As an RV’er for over 45 years I believe I can attest to the issues of setting up in a parking lot.
Firstly, as most of us know there are some who abuse the Walmart parking lots. The vast majority do not.
When travelling from point A to B, we are not in the need of access to beaches, pools, spas, meeting rooms, etc.etc. What is needed is a safe place to stay. There is safety in numbers, as in a Walmart lot.
I once did stay in a Walmart parking lot for a night. After my wife made two shopping excursions to the store it turned out be the most expensive free parking site that I have encountered.
My advice is to advertise that, in your local RV park that it has a limited number of sites with reduced rates for, and only for, overnighters. No full hook-ups, just power.
This may fill up his empty spots.