We have stayed at Boyd's on Stock Island which would suit your requirements.
https://www.boydscampground.com/ If their marina happens to be full, there are several within a short walking distance.
We have stayed at Pennikamp State Park which has a ramp and marina. Also at the KOA on Sugarloaf Key, but I understand that is still closed for repairs.
There were quite a few others, but we have not stayed at any of them. Generally speaking all of the RV parks in the keys are 1) expensive; 2) pack you in like sardines; and 3) are booked up far in advance.