Since Truma has started selling their products here in the US, I wonder if they might be something to move to for smaller RV models.
For example, their Combi Eco Comfort Plus model is a combined water heater and furnace, works on both propane and electric, and puts out more usable hot water than the typical Atwood or Suburban water heater. This furnace puts out about 20,400 BTUs on propane, and 14,300 on electric, using 14.2 amps at full tilt, 7.1 amps at half power. The furnace and water heater are separate, so you can run the furnace with an empty water tank, or run the water heater without the furnace having to be turned on.
The advantage of this is that there only needs to be space for one appliance that is smaller than a four gallon water heater. Plus, the appliance doesn't need to be mounted near a wall... just a hose with the intake and exhaust like most furnaces. It also costs less than the separate water heater and furnace.
I think this would make a great addition, mainly because it provides more usable space in the coach.