I've never been in a trailer so I don't know how cold it gets when the AC is on, but with my set up I can have the ambient temp of my girls cage at 80 and the ambient temp of my bedroom in the 50's. The black bulb I use puts off almost no light but enough heat to keep it plenty warm in medusa's plastic habitat. to help keep heat in you could also try taping up part of the metal screen that you mentioned.

If you are using a heat lamp be careful about how close it is to your cage I have mine pointed at the side of the tub (no way to hang it) and the surface temp of the cage wall is something like 96 degrees. I also did a test and hung it 2 inches from the floor and put a temp probe under it and the surface temp of the floor directly under the lamp was 109, so deffinatly watch how close the lamp is to your cage.