The closest thing I have ever encountered like this was if I was using the 30A or 50A side I would not be allowed to also use the 20A side for an Electric heater. They did say when I spoke to them about this as that is my first choice because I hate to listen to the loud blower motor on the Propane Furnace. They said it would be ok to use the 20AMP Service for outside low wattage items.
After hearing that I went ahead and used the extension cord to power up my electric heater inside the trailer...
Then marked that site off my favorites list.
Roy Ken