Your motorhome sholud have a "slider" type resistor block.
Take the slider apart.
Clean the bottom and top half of the slider with a wire brush.
Reposition the slider in it's original location, or there abouts.
That will fix your "low voltage." at the brakes.
The problem is the sliders are a "shunt." When the shunt becomes corroded, however slightly, it no longer makes connection. Therefore the entire resistor comes into the circuit, reducing the voltage excessively.