I would first work with the heater manufacture. Tank less heaters work with water flow. Not enough flow and they won't come on. Might be the thermostat is set too high and your attempts to get of comfortable water temp may not be enough to trigger the system.
Replacing the tank less is certainly an option considering both a tank or tank less have pretty much the same foot print but you'll looking at a bill of over a $1000 to make the change (cost of the new heater, new propane lines and associated labor at around $100 hour). Plus the original coach builder warranty probably won't cover the new heater which might complicate matters.