I see many inexpensive diesel fueled air heaters advertised (made in China). They are used for camping and small RV, Truck cab heating when parked.
Wabasto makes them also but they cost near $1000 compared to $150 for the Chinese versions.

My use is as a emergency back-up heater for my outdoor Endless Pool SwimSpa ( about 1100 gal. 14'x10')
I keep it going year round, covered (the top) until use and set to 95 degrees, I turn it up to 101 degrees if I plan to use it (heats 1 degree per hour). Its min. setting is 85 degrees.

My concern has always been a extended power outage where the electric water heater can't keep the pipes above freezing.
The Pool is enclosed in a insulated aluminum cabinet, the tub has closed cell insulation sprayed onto it.
My thought was a diesel heater that can be run on a 12v battery inside the pool enclosure. I have a RV parked in the yard with 180 gal. of diesel available. Keeping a 5gal can of diesel around for the heater would be easy and I have a solar battery bank available to charge any battery.
All I'm looking for is a way to keep the pipes above freezing temps in an emergency.Since the heater would be outside I don't think I would even need to vent it.

Does any one have experience with the Chinese made ( I know) heaters?

Yes, they are used in semi trucks for heat in the sleeper unit. Have a customer that uses a China one with great success. Although I would think that you need to circulate the water to keep if from freezing. I think it would take a long time for 1100 gal. to drop enough to freeze and you would just need a circulation pump to keep it from being isolated in the pump and pipes, therefore freezing.

Just need it to keep the outside of pool where the pipes and pumps are located above freezing.