I just installed one in my mobile home. It was an electric furnace in a small closet. All I did was built a tin box for the exchanger and moved the furnace up that amount. Lots of room to raise it. Not sure about the gas but I bet it's a lot of empty space I would start removing stuff and take a look. I would do any cutting underneath if you don't have too. And it actually is easy toi fix if you buy the proper insulation and belly tarp to install it. The insulation is available in 48" wide batts by as long as you want, same stuff as used on ceilings and walls of steel commercial buildings, belly tarp backing material is already glued to it. just cut the material to length and there is a 1x4 piece of wood screw nailed to the bottom of the floor joists that you can screw nail it to. If you do a search on utube for trailer floor repair there is lots of videos of what you can do to repair the floor insulation.