Sounds like you are planning to add two circuits.
You do know you only have to have on (the first) GFCI in a circuit to protect the entire circuit, right? You connect the source to the "line" and the remainder of the outlets to the "load".
You don't really want to put a 2nd GFCI on an existing one. You will run into false ground-trips.
The biggest problem is getting inside the walls to run the wires. Unless you are going to surface mount your cables.
Also, your basic assumption if flawed. You are going to run a cord to the pedestal 15 amp, a lot (certainly not all) campgrounds don't have a 15 amp outlet. Yes, many have a 15 amp 110, a 30 amp 110 and a 50 amp dual-110 (since it's not really 220). But I note a significant percentage of campgrounds that just have a NEMA 30-TT outlet and a single 30amp breaker.
If you have room in the rigs panel, I would add a breaker, maybe separate the Microwave to it's own circuit (that's how my last two rigs were wired, from the factory)
I would suggest that you get someone involved that knows both the NEC and why it states what it does. I'm saying this as both an Electrical Engineer and a (former) Electrician that spent 35 years working the Navy ETs and am reasonable familiar with their training (or lack there-of).
I'm not saying hire it out, just find a local friend that can guide you through the "oh ****s"