Have been getting scamp camp ready. One of our last tasks was to flush & fill the water heater tank. Hmm...didn’t fill automatically from fresh waster tank, so their must be a valve somewhere? Watched the scamp helpful videos, searched the manuals, checking the scamp outside and inside for some indication. Accidentally figured out that to fill the water heater tank, we had to open both hot & cold water kitchen faucets. It was filling!! Yeay—now to get flow from the water heater tank. Again, hmm... Rechecked everywhere for a discharge valve. Ugh. Finally, Gary looked at and touched every single thing in the shore power box under dinette seat until he found a slim, black-handled valve that we have determined is the inlet and outlet for the water heater. It was tense for awhile 
but we learned something. In case there are other first time scamp campers out there who are as simple as we are, I’m including two photos of the valve. 
Trying, but not sure photos are uploaded
Thanks so much for the heads up on this! We are about to de-winterize and are timid because this will be our very first time ever filling, flushing, cleaning, and all the above!! Any advice on de-wintereizing as a newbie?
Stephen and Cameron Freshwater
The trick is to use Freshwater. Either Mr. or Mrs (Ms) Freshwater will do but the boy is too young to handle the job.
LOL at myself.. love that name! What a perfect surname for campers!
Now on to the question... Drain antifreeze, then sanitize according to these directions. Fill and flush until smell of chlorine (and antifreeze) is gone. Flushing the water heater should be covered in the owners manual but note that most people bypass the heater when sanitizing because it is normal for some water to remain in the water heater tank, and therefore very hard to get all the chlorine solution out.
Yes the system is sealed and water / air tight, so you have to vent air from the water heater so it will fill. Open the hot faucet and fill the heater until water comes out. Failure to do this before turning the water heater on will damage it. (The holding tanks are not on the pressurized side of the system and have vents so that you can fill them)
As for the valve, I suspect this is a user mod and not OEM. But I cant tell too much from the photos. I even tried to enlarge them but that did not help. When adding photos on this site, first edit them offline and resize to 640x480, then when you upload and insert them, be sure to choose the option to keep original size. The site seems to default to a small size that is often about useless.
As for the valve, I know it is common to have a single shutoff valve on the cold water supply line going to the water heater. With the newer Uponor ProPEX plumbing Scamp is using, it is usually a circular valve that you turn. It might be ball valve with a handle in older ones. Perhaps that is what you have pictured. I have not heard of another valve on the outlet side of the heater, unless it is a full heater by-pass (for bypassing the water heater when winterizing).