The two switches are probably wired in series. In our SOB trailers over the years, there has always been a light switch by the door that turns on at least one overhead light. But the overhead lights in the trailers are all the same in that they all come with built-in light switches. So, in order for that light to work, both switches need to be on. Basically, you leave the switch on the light in the on position and use the switch by the door to control that light. Since you're re-wiring it, you could simply bypass the switch on the light if you want to, although that might confuse someone down the road. The wires from the batteries should also connect directly to the wires going to the front lights and fan (without going through one of the switches first). That way those lights and fan always have power to them. I'm assuming that the front lights and fan have their own individual built-in switches, right?