The only way I can see a tankless water heater being useful in an RV is with a recirculating valve, so the water that is used to warm it up is shunted back to the fresh water tank and not wasted.
Instead, I'd probably consider a Truma Combi system as a possible upgrade. It has less water, but heats it quicker. For an upgrade, it may be difficult, but for a new rig, it does the job of both a 20k BTU furnace and a water heater, so wish RV makers would use it, freeing up quite a good amount of cubic feet for storage or other tasks.