I'm using a propane burner at the moment, it's a lot cleaner burning that a diesel burner and the temp is very easy to regulate a bit like a gas hob, you can turn the gas down to an almost flicker, the diesel Burner is on full constantly no adjustment which makes it very hard to control.
But although the Burner is less bulky than a diesel hot box you have a gas bottle to fit as well. Propane is cleaner so you can fit the heater anywhere in the van but a diesel burner needs either a chimney through the roof or be next to the door..
You can use a portable propane shower like the window cleaners but it can't take the pressure but you can fit it before the pump so the pump pulls the water through the heater or if you use it with a truckmount you could use it to fill the water box to preheat the water