If you take the large wires on the two large terminals on the battery isolation solenoid off and fasten them to one terminal, you'll bypass the solenoid. If you now have 12v power in the RV with the shore cord unplugged, you'll know it's a bad solenoid. If still no power, then look for the large amp fuse or circuit breaker in the cable between the battery and the solenoid. Check for 12v on both sides of that fuse/circuit breaker.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI