Thanks for the tip! - if it is, it's probably a ground problem with the isolator I installed in my RV recently. I installed a new wiring connector on my trailer because the ground wire broke off a couple of weeks ago . I also installed a fresh ground screw into the trailer toungue to be sure when I redid everything.

I installed the isolator in my RV so the trailer lights will have their own fuse. While coming back from Tradewinds this January, I had one of my trailer side marker lights short out and it took out ALL my running lights on my RV as well as my antilock brake unit. I imagine the lighting system was pretty overloaded with all the marker lights on the RV before I put the trailer wiring in (there was no hitch on it when I bought it). I suspect that it was related but I also began to have an intermitant Engine Management Computer problem that kept getting worse and worse on the next trip and almost kept us from getting home (turned a 5 hour trip into 13 hours). After four days of diagnosing because the computer wouldn't give any fault codes, several engine sensors were needlessly replaced and I ended up finally replacing both the ECM and the Anti-Lock Brake Computer on an RV with 30k miles on it (CHA-CHING).

Four hours to install a $35 trailer lighting isolator unit? Priceless.