Think you're going to need a loss prevention / fire protection specialist here to write a report saying what it is you need and why.
If you have no "code" to follow then you look at the risk, the impact of a fire, what you can do to prevent it from spreading too far and then design accordingly. If the FD won't give you any guidance then give them something that someone has signed off on and let them come back to you.
If you have nothing to protect other than the RVs and maybe a few personnel to escape, then in my opinion you only need a limited or maybe no fire water. It's all about the impact on people and property beyond your fenceline. Inside it's your problem / risk.
Drain fuel from the RVs or remove LPG tanks when stored, maybe need to limit the number parked and create a few "fire breaks" and maybe have 15mins of water supply to a few hydrants. Maybe need to not have any within say 10m of a fenceline so third party gets hurt if it all goes up in smoke.
They call it "burn down" in the hydrocarbon industry and then you take on the risk if it all goes up in smoke. Your insurance company may have something to say about that though so try them as well to see how they would react?
I mean look at all those aircraft parking lots in the desert? I don't notice too many hydrant systems there.
Without double posting, there is an NFPA forum on this site so maybe search there or ask a similar, but not identical question in that forum. Its; under "engineering codes and standards"
I did a bit of search and NFPA 13 and 88 come up, like here
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