Typically, the heat exchanger in a RV water heater is to allow the engine coolant to heat domestic water while traveling down the road.
There is a thread here somewhere that describes installing a exchanger equipped water heater in a trailer, then using it in "reverse", to provide in-floor radiant heat.
I think that is a clever idea.
Jeff & Cindy
'09 27FB Flying Cloud
'91 350 LE MH