If money is an issue, and you want maximum bang for your buck, I'd probably go for a "Big Buddy" heater, the fuel filter (the element is sensitive to the oils in fuel hoses), and a propane bottle outside the rig. It can run for 100+ hours on a 20# bottle, assuming mixed low/high use. There are three caveats:
1: I have seen rigs utterly destroyed when a heater like this ends up lighting up bedding or other flammables. Best placement I'd guess would be in a metal baking tray on the stove, with a battery operated fan blowing air above it for circulation. The Big Buddy has a built in fan as well.
2: Ventilation is key.
3: Propane burned makes a lot of water, so that can end up an issue if not planned for.
Of course, there are other alternatives. There are people using wood and wood pellet stoves, but that is a big DIY project, although wood isn't expensive.
