Sometimes the best way to work in that compartment is to do it from the bottom. If the battery tray rolls out, sometimes there is more access from under the coach with the tray slid out. You may even have to remove the batteries completely. If you do, take a few pictures before disassembly to help when it comes time to put things all back.
One more thing you can try to help isolate the issue - With the master toggle switch disconnected as you tried, lightly "tap" on the solenoid with a small hammer or wrench being very careful not to short out the leads. Sometimes the tapping will cause it to disconnect. But the "on" trigger must be turned off or it will not release.
If tapping works, replace the solenoid when you can. It may start to work again, but it only is a matter of time.
2014 Seneca 37TS
Toads: 2014 Ford Explorer Sport or 2013 Ford Focus ST