OP One technique is to route the 3rd AC wiring to a male and female plug in a storage compartment and then continue on to the 20A RV panel CB. Now use a heavy construction cord (10 ga) to connect the AC to a 20A pedestal plug when 50A is not available. This is a common technique and there are posts on this mostly for a 30A rig with 2 ACs.
However a 30A and 20A pedestal may/may not be wired to handle a 50A load. You should be monitoring to monitoring the voltage to be sure a pedestal is OK. Another pedestal in the same CG that is closer to the CG could also be helpful.
I added a second circuit for ceramic heaters when we're on a 30A pedestal. By default it's plugged into a dedicated 20A RV circuit. But when cold it gets plugged into an available 20A plug.
The dealer is right about a 15A plug. However many pedestals have a dedicated 20A plug.
Lots of fun. :B