Coincidentally, I just ran a test of my Chinese diesel air heater yesterday. It sounds like you're leaning toward other options, but here are the results anyway in case it helps somebody make a decision:
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Keep in mind that there are way too many variables to be able to get the same results on any two buses. For context here are some of the details from this test:
Outside air temp was fairly constant at 12-15°F throughout the test. There was very little or no wind throughout the test, which makes a HUGE difference. The bus was in 90-100% shade the whole time. For the first 3.5 hours, the bus was empty and closed up while I monitored the temp from inside the cozy house. After that, I went out to do some painting and the door was opened several times. I'm assuming that my body heat made up for those small temperature losses, but I have no way of truly knowing.
The bus is 40', with an internal height of 75" (no roof raise). It's a work in progress, but most of the primary insulation is in. It has 1.5" spray foam in the ceiling, 1.5" EPS foam boad along the lower walls, and half of the windows are insulated over with 1" XPS. The remaining windows are stock single pane and were not covered for this test. The floor is approximately 1.8" of wood at this point (not much insulation value). The entire front/dash/entry door are uninsulated.
The heater is a knockoff of the Espar D5. It is a MaXpeedingrods brand 5KW (~17k BTU) model from Amazon. It is mounted about mid-way down the bus and shoots hot air fore and aft.
In temps down to about 25-30°F, I turn on the heater about 45 minutes before I head out to the bus and it's already up to my set point of 68°F by the time I get out there. As you can see from the graph, it took much longer to heat up this time.