Hi,
We used propane for space heating and cooking on our PM conversion. If we ever put a hot water tank in (unlikely), we will use it for that as well.
Ours uses a regular 20 lb BBQ style propane tank that is mounted inside the van in a strong, sealed compartment that is under the bed. We lose a bit more than 1 ft by 1ft of under bed storage to the tank.
The plumbing is very easy. Is 3/8th inch diameter copper line with flare fittings. There is a total of maybe 10 ft of easy to run tubing and one Tee with one branch to the cooktop and the other to the propane furnace. It could not be much simpler or easy.
When you are low on propane, you pull into any one of what seems like a million propane BBQ tank exchange services -- almost ever gas station has one these days-- even grocery stores have them. You give them your old empty tank and they give you a full one. Less than 5 minutes, and you can combine with a regular stop for gassing up the van. While a 20 lb tank is small by traditional RV standards, A well insulated PM does not take a lot to heat it, so even a relatively small 20 lb tanks lasts about a week with space heating (forever for just cooking).
The furnace is an RV type propane furnace. These furnaces are pretty cheap and have been incrementally improved over the years. They are basically the defacto standard for the RV world.
The furnaces have all the same sort of safety features your home gas furnace has, and they vent combustion products outside and get combustion air from outside. They are compact, and the smaller ones do not require ducting to distribute the heat, but they do require an about 3 inch hole in the side of the van for the coaxial exhaust vent and combustion air intake. Ours is installed in the galley cabinet. They are in the $500 area. So, the whole bill for propane tank, plumbing, furnace, and cooktop can be well under $1K. You can add a small RV propane hot water tank for about $300.
Our last RV had an underfloor tank, and while it was OK, it was hard for the propane guys to fill and had a couple component failures due to constant exposure to weather and road junk. Bear in mind that you will probably want a grey water tank under the floor, and this takes up some space.
Mind numbing detail on our installation here:
http://www.buildagreenrv.com/our-co...van-conversion-installing-the-propane-system/
Gary