In my camper van, there is a 5 gallon Manchester horizontal "van style" propane tank mounted to the frame under the sliding side door. The van style has the fittings lower on the tank, and has internal tubes that rise up to the top to be above the vapor line.
When I got the truck, I replaced the regulator with a new two-stage. Cost about 20 bucks.
From the regulator, there is a short - about a foot - rubber line with flare fittings on both ends that connects to 3/8" copper tubing. I replaced that too. Had the local propane yard make me up a new one. 15 bucks.
After that, it's all just copper tubing and flare fittings with a few tees to feed the stove and furnace on the starboard side, and the water heater and fridge on the port side.
Since it's all low-pressure after the regulator anyway, as long as the copper is strapped and doesn't rub anything, it will be fine. Or, at least is has been fine since the van was converted some time in the late '70s.
You could also use black iron gas pipe if you wanted.
Current: 76 E-250, bubble-top, self-contained|couple of old Yamaha enduros
Previous wheelers: 41 Willys|78 FJ40|78 Bronco|84 Bronco|74 Ramcharger|78 Ramcharger|79 D150 PowerWagon|77 D100|79 D400 dually, converted to 4WD, utility bed, 10' Lance|75 Westy|69 Scout, RHD|bunch of others|bunch of bikes|couple of boats|couple of motorhomes|blah blah|so what|not my idea|just doin' what I'm told|wank wank|this space for rent|candy is dandy|but liquor is quicker