when running opposite phases, the neutral would only take the difference
so with a 30 amp one side and 20 amp on the other side the neutral would only be pulling 10 amps
That is where my thinking was flawed.
I can get by with a double 30A in the combo box because the breaker only protects the run. The pedestal has a 30A and 20A breaker protecting the outlets.
Reason for a 20A plug is it is GFI and will be used for other things. Only a small truck camper will be using it, not a large RV with high loads so the 20A will be enough.
Thanks for the quick response.