Spacing between service panel and RV sub panel (both on same outdoor freestanding plywood/post)
The panel can be mounted directly below there is not a minimum height or spacing for the service panel. There are maximums  6’7”  for the max height of the disconnect. You could have the panel touching the ground (not a good idea).
The only exception to this is if close to a body of water that floods then the panel has to be 24” above the normal high water.
Note I have worked in local areas that have had 18/24” minimum heights for the bottom of service panels outside but this is not in the NEC.
It is best to check but the standard code a minimum is not listed
The 2 boxes could be connected with feed thru bushings or nipples to whatever you want no money max on the separation either.