While looking RV hot water heaters I ran across a few tankless ones at PPL Motorhomes. Prices are high on them, but they offer a built in option for a trailer, and they are designed for vehicle or trailer use. The BTU outputs look to me to be lower than the home units some have used.
Atwood has 45,000 and 50,000 btu models. They fit in standard RV waterheater places. http://www.pplmotorhomes.com/parts/rv-water-heaters/rv-ondemand-water-heater.htm
Girard Tankless RV Water Heater is adjustable from 18,000 BTUs to 36,000 BTUs, 12.5" x 12.5" x 15.5". Needs 12VDC for blower for combustion air and electronic ignition. Boasts of 90% efficiency heat exchanger.
http://www.pplmotorhomes.com/parts/rv-water-heaters/rv-tankless-water-heater.htm
PreceisionTemp also makes them, and from a press release it looks like they may make the Atwood units. They have the RV500 unit on their site: http://www.precisiontemp.com/pt_rvmd_rv500.html but I've seen a press release for a RV550 unit that doesn't need to use the standard RV hot water heater door. I can't find specification for it. RV500 13.5" Wide x 13.5" Deep x 14.5" High. Weight: 35 pounds. 12 VDC at 1 amp. Operating range is 9 to 15 VDC.
56,000 Btu input – 44,000 Btu output at maximum burn at 8.5 WCI manifold pressure.15,000 Btu input – 12,000 Btu output at low burn at 1.2 WCI manifold pressure. (85°F temperature rise per gallon per minute continuous flow at high burn). Propane input to output numbers say 78% to 80% efficient heat exchanger.
Atwood has 45,000 and 50,000 btu models. They fit in standard RV waterheater places. http://www.pplmotorhomes.com/parts/rv-water-heaters/rv-ondemand-water-heater.htm
Girard Tankless RV Water Heater is adjustable from 18,000 BTUs to 36,000 BTUs, 12.5" x 12.5" x 15.5". Needs 12VDC for blower for combustion air and electronic ignition. Boasts of 90% efficiency heat exchanger.
http://www.pplmotorhomes.com/parts/rv-water-heaters/rv-tankless-water-heater.htm
PreceisionTemp also makes them, and from a press release it looks like they may make the Atwood units. They have the RV500 unit on their site: http://www.precisiontemp.com/pt_rvmd_rv500.html but I've seen a press release for a RV550 unit that doesn't need to use the standard RV hot water heater door. I can't find specification for it. RV500 13.5" Wide x 13.5" Deep x 14.5" High. Weight: 35 pounds. 12 VDC at 1 amp. Operating range is 9 to 15 VDC.
56,000 Btu input – 44,000 Btu output at maximum burn at 8.5 WCI manifold pressure.15,000 Btu input – 12,000 Btu output at low burn at 1.2 WCI manifold pressure. (85°F temperature rise per gallon per minute continuous flow at high burn). Propane input to output numbers say 78% to 80% efficient heat exchanger.