There's no way one of those chintzy on demand heaters would be a good fit for this task.
You'd have to bypass the temp and pump controls to make it work.
If you can find an old Webasto Thermo Top C, that is probably the cheapest and most common hydronic heat source that will put out the btus and temps you need. Every school bus in CO got one in a program a few years back. They use about 50 watts when running and can put out over 17000 btus on about a gallon of diesel every 7-8 hours or run on low at 8000 btus for over 10 hours on a gallon. 8000 btus should keep a modestly insulated bus warm into temps in the 20s.
You could run the baseboard radiant heaters on a loop with the webasto. If you added a 30 gallon tank to the loop, you could run the webasto for a while to get everything up to 170* and then coast for a few hours. You would save a good deal of fuel doing a setup like this. Then, when it's time to start your engine in the cold, just open a valve that loops your bus's engine into the hot stuff and it will fire up like a dream.
That's my plan anyway. I only need around 5000 btus to keep warm on all but the nights below 0. If I heat a 30 gallon tank of water to 180* and can use heat from it all the way down to 100*, I have over 17,000 btus stored up. If I can release that over the course of 4 or 5 hours, I wont have to run the webasto at all. I think it could be worked out to run it for about 4-6 hours per day (morning, evening, and night) and save the heat to keep warm. 6 hours at .16 gallons per hour is just about a gallon per day of diesel. If you burn the off-highway stuff it's a good deal for push button heat.
Of course, you could run it on low, but where's the fun in that.....
Patina enthusiast and professional busman