That does change things abit but doable...
This is how the standard 50A to 30A Adapter is wired up. It just grabs one leg of the 50AMP source to run your 30A Distribution Panel.

Since only one leg of the 50AMP service is connected than there will not be any 240VAC present on the trailer side.
This shows what voltages are present on a typical 50AMP Service

All photos from GOOGLE search
We do this alot at camp grounds as most of the folks are all on the 30AMP wring and sometimes loading down the camp site pretty bad. We get better voltage by plugging into the 50AMP side.
The only downside to doing this is the shore power cable from the pedstal to the RV trailer may not be heavy duty enough for the 50AMP service if a short occurs in the shore power cable between the trailer and the 50AMP connection. i.e. the 50AMP Pedstal will provide protection for the 50AMP service and the trailer 30AMP Distribution Panel will only provide protection for a 30AMP Service.
I would think your best bet would be to run another extension cord plugged into the 20AMP service on the pedstal just for your space heaters. The problem of course is how to get the additional extension cord inside the trailer and NOT have a large gap somewhere for cold air to get into the trailer hehe... I have no problems with my POPUP doing this but on the 5th wheel I can come in thru the storgae bay door and push some foam around the cable to seal it off pretty good. My 5th wheel has the dirty clothes drop which give me a hole up into the trailer to run my extension cord.
Roy Ken