Some R.V. Parks charge according to the size of rig that you have. Most all of them will ask what size of rig you have when you reserve a site so they can match you up to an appropriate site for your rig. If you want a certain site number when you reserve a site, tell them that, but you may also pay more for that site.
When I was buying my last RV, I rented an Rv site for 5 days. The first 2 days, I was parked in the site with my old 5th wheel. The last three days, I was there, I was in my new rv that was 4 ft longer. They re-adjusted my rate for the larger trailer at the same site. I may not have liked it, but I was taking up more space at their park.
If you want to park wherever you want, perhaps considering a state or us forest camp ground would work best for you.