I assume that the 2018 Tacoma still has the same Sheet Molding Compound (SMC) bed, so the installation of the rails would be the same. Here is how I did it: 

I hope this helps. I made several long trips last year, about 8k miles in all, and had no problems with this setup. (Of course, a disclaimer is in order: I have not run into any extraordinary circumstances, accidents, storms and such, so the margins of safety are not well known. But this applies to all automobiles on the market anyway, in my humble opinion.) The plates I had made for this setup were about $30, the friendly custom truck maker must have used some leftover pieces of steel; I expected to pay more than double that amount. 
Good luck!

Installation of Scamp 5th wheel hitch in the 2013 Tacoma SR5 with towing package, Double Cab, Short Bed. 
1. Purchase Scamp "New Style Hitch", $495, S/H $90, came by UPS in two packages. Mostly Reese parts, with the 2 inch ball for the Scamp on a custom made plate. Also included are 1/2 inch bolts, lock nuts and backup plates. 
2. The hitch rails are just a bit too long, cut off 3/8 inch from each end of both rails. 
3. Assemble the hitch legs to the cross member in the lowest position of ball. The bed is 32 inches from the ground, the ball ends up a little higher than the 40 inches that is recommended in the Scamp Owner's Manual. 
4. In addition to mounting through the bed floor, I chose to use the four front bed bolts for mounting the rails. When the front side of the front rail is attached by the front bed mounting bolts, the ball ends up just in front of the axle and is a little less than the maximum 59.5 inches from the corner of bumper (again, recommended in the Owner's manual). 
5. The bed bolts use T55 TORX driver bit. The factory installs them *very* 
tight. Also, the 12mm bed bolts are long enough to be reused. The two front bolts need to have the integral washer removed. Carefully file the ridges on the shank to slip the washer off. (If you have to put them back, note that the hole has a chamfer that goes against the underside of the bolt head! Keep the same orientation.) 
6. Two pairs of steel plates were made and total 8 holes .50 diameter drilled in the rails (four in each). The special plates are 3/8 inch steel, which is exactly the depth of the ribs in the bed, so the rails rest on them when bolted down. 
7. The front plates are 2.00 x 6.00 inches, one .50 in. dia. hole, 1.00 inch from end, 1.00 inch from side (on CL of plate). 
8. The rear ones are rectangular 2.50 x 11.00 inch, with a corner cut-out 1.13 x 9.00 to make them L-shaped, one .50 inch hole is drilled in the foot of the L, 1.00 from end, 1.13 from side (centered on the foot of the L). Two more holes will be drilled in each later. 
9. Lay the rails in the bed. Carefully measure the distance between the front two bed mounting bolts and mark the location of holes to drill in the rail. Align them with the square holes already punched in the rails. Drill the holes, take the bed bolts out and lay the rail in place. Torque the bolts lightly in place, they will need to come out again. 
10. Take out the middle pair of bed mounting bolts. Mount the L-shaped plates and line them up well with the ribs. (If necessary break the edges of those plates so they align well with the features of the bed floor and lie flat.) 
11. Centerpunch for mounting holes in the rear side of the front rail, aligned with the third ridge from left and the third ridge from right. The underside is clear of ribs or other features, and reasonably accessible. (Ideally, mounting this rail is a two person job.) 
12. Assemble the hitch in the bed. 
13. Centerpunch for holes in the rear rail where the line going through the centers of square holes in rail edges intersects the centerline of the upright of the L plate. The distance between the two holes on each end of the rail should be same as the distance between the square holes in rails and the round holes in the backup plates supplied by Scamp. Pre-drill in those four locations with 1/4 inch drill and drop in a pin, one at a time in the holes drilled, so parts do not shift on you. Mark the L plates (left, right) and the rear rail end (left-right). 
14. Take it all apart and finish drilling the two holes in the rear side of front rail. Then finish drilling the four holes near the ends of the rear rail, and the two holes in each L plate. Assemble it all in the bed, replace the bed mounting bolts (loose), align the holes in the rear rail and both L plates (drop in the bolts to keep things from shifting). 
15. Drill through the bed for two bolts in the front rail and push the bolts through. (Use a short drill or be careful not to go more than just through the bed.) 
16. Drill through the bed for the four bolts in the rear rail and drop in bolts one at a time, to maintain alignment. These bolts come through close to the outside of the bed, visible in the wheel well, next to a rib. 
17. Use the supplied backup plates and lock nuts to attach the rear rail. (It may be necessary to shorten one end of the plates to stay away from a rib. 
18. Cut one of the backup plates with two holes in half, it will go over the front rail bolts. Now for the fun part: To get better access to the front rail bolts under the vehicle, drop the spare and crawl under to get oriented. There is a lot of room there, acutally! Put the half backup plate over the bolt and start the locknut. With two people, it is a snap. If alone, use visegrips to hold the lock nut, get out and tighten the bolt from the top. Make sure you can get the pliers out easily! The left side is more crowded with cables and lines, but a box wrench (3/4 inch) and visegrips can do it. 
19. When all ten bolts are tight, you are finished! 
Note one: I added a pair of 24 inch .15 x 1.5 inch steel pieces connecting the front and back rails to help distribute the towing or braking forces. The play between the ball support and the rail slots is so big that it is one rail or the other that carries the load first and the bed must give some before the other rail picks up some load. The straps help transfer the load to both rails. 
Note two: It is good to break all edges on all the plates and backup plates used here. 
Note three: Connecting a brake controller to the wire harness was a nightmare because the connector for the brake controller pigtail was extremely difficult to find and pull from behind the wire harness. Better assembly practice at the Toyota factory is in order.

Sorry, I do not have the info on how high the ball coupler is from the ground. It also depends on whether the Scamp has the axle lift. The Scamp people (Eveland's) should be able to provide that information, it may even be posted on their web page. My Scamp has the axle lift and the adjustable Reese bracket is on the lowest setting.