Originally Posted by Thehobe1
You can buy an extension cord with a GCFI built in to the cord. This is the ONLY way I would consider it OK to use your separate outlet....just in case.
There are several safe alternatives to add GFCI...
Sounds like the OP is plugging into a 110/20a circuit on pedestal. By code I expect these are already GFCI protected (as they are outdoor 110 15a/20a outlets)... but certainly don't need to trust that...
A GFCI extension cord, or GFCI plug as the only (or first) outlet inside would add protection.
Note that most outlets in the RV are NOT protected by GFCI - typically only the exterior, and anything within 6' of water (kitchen counter, bathroom). These are where GFCI earns its keep.
If not in proximity to water inside, and using the waterproof marine outlet to pass power in - while it can't hurt to use one, I wouldn't go as far as saying it is required...