Bottom line, you should never see very much voltage drop between the battery
and the load, regardless of what the load is or where it is. If you are seeing low voltage at the load then the wire or something else in between is dropping the voltage. The only way it can do so is high resistance. Resistors cause heat when current flows through them. Small wires can be resistors. Insulation can burst into flames from heat, or melt and expose neighboring things to hot wires.
Doesn't sound like a good thing to me.
I would absolutely believe that wires from the battery are too small. High resistance crimps, lug nuts, fuses. Prepare to rewire, I certainly am.
BTW I have an old 1995 Scamp
19' which will almost certainly be treated to an entirely new wiring system.