I have a gas van, and I decided to go with the Webasto Airtop 2000 STC from Russia. Cost was obviously a driving factor, but like others I was unconvinced that the Chinese-direct heaters would meet my expectations. I'm certain that anyone shopping for one of these has read the instructions for switching to high altitude usage from TechWebasto using a rheostat:
Webasto only rates this heater to 2200m (~7200ft), but the comment about turning the controller to 8 o'clock for altitudes of 10k+ (plus the instructions of adjusting based on CO output) indicates to me that the setting is a continuum.
The next larger in Webasto's line actually has a built-in altitude sensor, but it's considerably more expensive and my understanding is that the high-altitude version (5000m) is OE-only. I also reasoned that the 2kW heater will be running "harder" to maintain temperature than a 4kW heater, and thus less likely to coke up.
All TBD though, just showed up a few weeks ago and haven't had time to install it yet.