That explains why they do.. I have been to some (Well ONE) park that had a post-office box station.. In this specific case it is a bank of outdoor post-office type mail boxes that could be used by Long term residents.. This is similar to an apartment house mail box wall but designed for outdoor use.
But I've only seen this one place.
What I use is a 3rd party postal box... My address is in the form of 1234 street #123 (not my address but the form)
1234 Street is the store 123 is the box number.. The UPS store often is this type of store...
What's more this place is a "Store and forward" service,, I call them up in about a week and even though I'm 800 miles away, a few days later I visit General Delivery (now that I have a car) or the guard house (Before I got a car) and pick up my "Care Package" Very good for RVers.. the boxes here are I think a couple dollars a month more than the Post office..
you can also go to the post office and see if they have any boxes.
DMV, in many states, will NOT ship to a PO box and of course anyone using UPS can not either but if you contract with one of the 3rd party stores that has an 'Agency agreement' like I do, they can also accept UPS, FED-Ex, DHL and such.
Very handy.