I dont know if they make a LP(or gasoline) powered Espar. But, if they do, that would definitely be your best bet. Then, plumb hard lines to the rear of the truck,(unless you already have rear heat). Use 5/8 rubber up, into the cab, and tie into PEX that is built into your sleep platform.
You will also need a way to limit temp to 100, or less.
Possible, of course....
I would stay shy of radiant. Snag a heater core from somewhere. Make a box to house it, and get a blower or low wattfan(s) in there. Operate it using temp if you like. This way, you eliminate any need for mixing valves, etc, as nothing that gets to the coolant temp, could come in contact with your skin/sleeping bag etc.
The problem with a removable source of heat, especially involving liquid as the medium, you introduce more possibility for leaks, you need to bleed it each time you connect it. You need to add a pump. You need some sort of a high limit, or energy cutout.
Chase