7 PIN WIRING CONTINUED

So, when you say the lights work correctly how did you check them, with the motorhome off?
If so and when you start the motorhome it blows a breaker?
A wire is being powered by your ignition and whichever one it is it is going to ground.

Lights work correctly motorhome running. Fuse blown.

Ground wire from the 7 pin ?

I might be out in left field, but I'm wondering if maybe the installer of the 7 pin plug on the motorhome used the same circuit that powers your slideout to power the charge circuit that feeds the trailer plug (IIRC that would be a circuit that was hot only with "key on"). I'm also thinking that your motorhome and/or trailer might have the ground and charge line pins crossed, so that it dead shorts your charge circuit when the trailer is connected. Another similar scenario is a bare spot somewhere in the charge circuit wire on the trailer is causing it to short to ground.