Scamp now puts the Girard GSW-2 in all their trailers. Most if not all tankless systems are made so they swap out with standard tank heater sizes very easily. I found some dangerous misconceptions regarding tankless systems when it comes to winterizing. The Girard line comes with an antifreezing circuit that at 38° will keep the unit from freezing. This only works if the main power switch on the unit is ON, if the unit has LP gas, and if it has 12 volt DC power. It is made for camping and not for long term storage.
Another issue regarding tankless heaters is the heater bypass valve. Tank systems would use a large and expensive amount of antifreeze during winterization to fill their tanks. This bypass plumbing is eliminated in factory installed tankless systems. A tankless water heater only uses 1 or 2 cups of antifreeze during winterization.
The first issue discussed is properly understanding the antifreeze circuit. Use it for camping only. If used for long term storage you must leave the battery in; you must keep enough gas in the tank; and you must keep the shore power connected to keep the battery charged (or charge the battery otherwise). Girard says the power switch on the unit is for storage. Turn it on at the beginning of the season and turn it off for storage. The antifreeze circuit is turned on and off this way. The control head inside the camper is used for all other heater adjustments, power, settings, etc.
You can safely fill the unit with antifreeze, and you want to... winterize it and fill it like all your lines elsewhere in your trailer. Filling in and out lines, hot and cold, etc... The antifreeze circuit only runs with the main power ON. You don't need or want that operable while winterizing with antifreeze liquids.
I hope this is helpful. I found lots of people saying tankless systems did not need winterizing. This couldn't be farther from the truth. I've used both systems and the tankless system is so good that it will take over eventually. If you replace your tank heater with a tankless one, keep or remove the bypass plumbing and valve as you see fits your needs.