I thought I would give you an update on this. I finally tracked down a company that provides this kind of equipment.
Basically, the final configuration is as follows (all parts are UL certified and conform to code).
A single 50A (from the pedestal) cable splits into 2 cables (30A and 20A). I run 2 x 30A extension cord from these into 2 x 30 to 20A adaptor cables and from there into 2 x 20A circuit breaker receptacles. So essentially I have 2 additional circuits protected by 20A circuit breakers.
On each of these new circuits I will have one 20A GFCI protected outlet and further along the circuit one double outlet. All this will be on 12 guage Romex.
I will have one 1500W heater on one circuit and two 500W heaters on the other circuits.
On the existing main 30A supply I will run one 1500W heater and one 750W heater. This should be enough to keep the RV warm internally and protect the basement compartment pipes/tanks from freezing.
This is all available from powersolutions (Bill Nemecek) at email@example.com