I've been looking into heaters more and found some good info.
If you want to heat with gasoline it is possible but it come at a price. The only one I found is a Espar at 13,000 BTU's (probably to big for a teardrop) at about $1,600USD. If you are going to use it above 5-6,000' add another $300 for a high altitude kit.
Diesel heaters will build up carbon above 6-7,000'. The Espar has the high altitude kit that will give you up to 4'000m a little over 12,000'. The Webasto can be adjusted with flashing the board but they say max doing that is 7'000'. Espar and Webasto are in the $1000 range.
I haven't been able to find out if the cheap clone diesel heaters have any adjustments for altitude. My guess would be no.
I have two emails to Propex but haven't heard back yet. I have read that 10,000' it worked fine. Very cold temps (0 degrees) and altitude were problems.
Here is a good thread on heaters and altitude. It is a Westy forum and they really put a big value in the one fuel. I don't know if it applies to us with trailers as much.
https://www.thesamba.com/vw/forum/viewtopic.php?t=608699&postdays=0&postorder=asc&start=0&sid=8f8184960e44fc15598f4cd78f5c1c28There is a heater like the Propex coming to the US next year that looks nice Truma VarioHeat eco. But they look like they will be $1100 without installation kit.
Bottom line, if you want to stay toasty warm at 10,000' or above you are going to have to shell out some cash for it.