I would pick a RV30A-15A DOGBONE style long adapter like this one from WALMART...

Don't mess with these small round adapters as they will get hot on you and do damage to your 30AMP Shore Power Connector..

You will be able to run most of your 120VAC items inside the trailer but anything high wattage will trip your pedstal 20AMP Breaker. You can run the high wattage items but only by itself with most everything else turned off.
I also have one of these RV120VAC VOLTMETERs plugged into where I can get a good glance at before turning on something like the Air Conditioner. This meter scale has the SAFE ZONE printed in GREEN. If the 120VAC AC voltage is outside the GREEN SAFE ZONE you should not turn on the air conditioner.

Roy Ken