How should I wire RV hookups?
You can share ground, but I believe you'll get into code issues if you share the neutral, so you'll need 5 wires (Hot 30 Neutral 30 Hot 20 Neutral 20 Ground 20&amp;30) even though the neutrals probably could be shared from a purely electrical point of view.

Ah, wait - you're saying you'd put a 30A 220V sub-panel on the post, so you'd just need the 4 conductors (L1, L2, N, G) to feed that, and then you'd run the receptacles from that sub-panel. Should work and pass code, don't know that it will be cheaper given the need for an exterior-rated sub-panel and a breaker to feed it. Wire is expensive, but maybe not THAT expensive.

Personally, I'd run 1" schedule 80 conduit and pull individual wet-rated wires rather than run a cable. But that's an opinion