When I hook up my Tow vehicle lights From my motor home . I get turn signal and running lights but only one side brake lights What could be the problem? I have tried every thing I know. Thanks

Try the 4 way flashers, walk around vehicle, see which are lit to determine if it is a bulb, wiring, or signal head/combination switch?

I think the bulb can be internally faulty or it can be that holder probably has corrosion in it. You should try to swap the lighting boards because if the fault moves then it is the board that has the problem but if it doesn't then the problem could be in the wiring.