Catalytic propane heaters discharge the products of combustion (CO2 and H2O) into the heated space. Oxygen depletion is a health issue. Condensation can also be a substantial issue. Even the smallest are sized much too large for the small volume of a car (both in terms of heat output and required clearances). The car is likely to become unbearably hot in less time than the heater's warmup cycle.
There are closed combustion forced air systems that use LP that offer similar advantages to diesel/gasoline systems like the Webasto (no oxygen depletion, no humidity increase) [1]. The LP systems generally have better high altitude performance because they operate on vapor pressure rather than utilizing a fuel pump. There are a lot of places in the US above 5000'. A downside is LP has a much lower energy density than diesel or gasoline so the cost per BTU is higher.
LP catalytic heaters also can leak propane and everyone who uses one is recommended to keep an rv propane detector nearby. I haven’t seen these forced air products before, thanks so much for bringing those to my attanetion!
There are closed combustion forced air systems that use LP that offer similar advantages to diesel/gasoline systems like the Webasto (no oxygen depletion, no humidity increase) [1]. The LP systems generally have better high altitude performance because they operate on vapor pressure rather than utilizing a fuel pump. There are a lot of places in the US above 5000'. A downside is LP has a much lower energy density than diesel or gasoline so the cost per BTU is higher.
[1] https://www.propexheatsource.com/