I like the curtain Idea. When I was driving my 64 truck. It sure was not warm in the winter. But it can also be blamed on that someone removed the insulation from the front cowl.
I like to find someone who makes those insulated panels to replace the one missing. Most wreck vans I find in wreckers or parts vehicles. these are all tore or dissinegrating. You see where people rest the toes of their feet against them.
I have seen rear auxilary dome lights and spot lights.But not a Hot water heater. My manuals show a rear gas fired auxilary heATER FOR WARMING the rear half of vans.