Why are you changing to a tankless water heater? They do not work well in northern areas, where the incoming water can be around 45F. I guess in Texas the incoming water is closer to 65F?
If you have 'free' electric, or low cost electric, use some electric heaters. I was able to safely use 3 of them in my 30 amp RV in the winter of 2007 in Portland OR. It was routinely 26F outside, and I could stay toasty warm inside. One of the 3 heaters was plugged into the power post directly via a #12 gauge extension cord ran through factory installed holes into the basement, and out the basement compartment door.
My electric cost was only $0.10 per KW. So 22 KW is $2.20. This will produce about 80,000 Btu's of heat. About what burning 1 gallon of propane (95,000 Btu's) will produce inside (considering heat exchanger and flue losses). Check your local electric rates, might be cheaper to go all electric.
If you can afford to replace the rooftop A/C with a heat pump, it can collect about 80,000 Btu's (in 5 hours) for only about $0.70. This is so much less expensive than propane or electric heaters.
What I did when living in my RV and taking long showers is buy a tankless L5 water heater from Amazon for $130. It is a VERY basic unit, with little more than a heat exchanger and ignighter, and high temp limit switch to shut off the burner at 120F. So I could use that to fill my fresh water tank at the rate of about 1 GPM at 60F temp rise. Once the 100 gallon tank was full, I would use the water pump to take really long and hot showers!
The tankless L10 is rated at 10 liters per minute (about 2.5 GPM) and what I would get next time to speed up filling the 100 gallon tank).
I used the little L5 to fill my above ground pool with warm water too. Only have to remember to keep turning it off and back on every 20 minutes. It helped 'warm' the pool, but is not a pool heater. . .
Good luck!
Fred.
Money can't buy happiness but somehow it's more comfortable to cry in a
Porsche or Country Coach!
If there's a WILL, I want to be in it!
I havn't been everywhere, but it's on my list.
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