1986 chevy p30

I have a problem with my van, a winnebago motorhome. The running lights will not work, fuse ok.when I have one interior light on and then pull the headlamp switch, the interior light goes out. it will come back on when i turn off the headlamp switch after a minute or so. power coming into headlamp switch. Charles Isles

Motor homes are tough. But here goes. I am thinking a ground wire but I would not have any idea where to start looking for it on that. I have had problerms with the switches also. JHH

yep start looking for a bad ground. I would start at the tail lights the work my way forward. Electrical problems su*&.