There are a few "Suspects"
Step one is obtain a 3LT (3 Light outlet Tester) Plug it in to a non-inverter outlet anywhere in/on the RV.. NOTE non GFCI is also good but may not matter.
You shoudl see two GREEN lights on most I'm guessing you will see ONE. (Showing open ground) (Note not all use green lights).
And if your unit has ONE red light it should be dark But that's suspect 2
Once you have the results we can assist you.. You can also do these tests with a multi-meter but for 10 bucks. a 3LT is nice and very very fast.
Get a 15-30 "Puck" type adapter (not a dog bone) and when you pull into a 30 amp site plug the 3LT in via the PUCK. in fact leave it plugged into the puck for storage.. Quick check of park
