i have the corwoods easy heat system, it is really simple to put in, you put it after the di and before the hose and it connects to a gas bottle costs between 1-2 pound a day to run and does heat up the water as hot as you like.
you put it near the back door if open, or you can install a mushroom vent in the top of the van, mines just near the back door.
I took mine out after a while just to use int eh winter because i got sick of connecters flying off etc, however, for the winter they do keep your hose from freezing up, 20 degrees higher than the ambient temperature is what you want in the cold