I would have the propane heater setup for backup - set to a low 40-50 Degree setting.
We can get away with using two 1500WATT OIL-FILLED heaters on our 30AMP setup but never set high than than 3 on the heat setting. Running high setting ends up tripping our breaker.
Your RV30A to 50A dogbone Adapter will feed both legs of the RV Trailer Power Distribution ok but just be aware there you will not have 50AMPs of service available. You will have to watch what is turned on at the same time...
NOTE the two HOT LEGS are strapped together using the standard RV30A-50A Dogbone Adapter providing 120VAC to your two zones in your 50AMP Distribution panel...

Typical two zone 50AMP Trailer 120VAC Distribution
Photos from GOOGLE search
Another thing we do that works out great for us is to run an additional extension cord out the 20AMP Pedstal COnnection just for our space heaters. These will be on the 20AMP Pedestal Breaker and NOT be affected by anything you are doing on the trailer breaker panels.
AS always - be sure to check for heat build-up when using adapters.
Roy Ken