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.
2017 VW Golf Alltrack
2000 Ford F250 7.3