In my experience most DCs won't let you park there before your appointment or after you load or unload, many have a lot outside, but not all, and some have parking on the acces road, like the Family Dollar at Maquoketa, IA.
I have been to lots of DCs over the past 7 years and onsite parking is not allowed more times than it is allowed. The newer, bigger, places are less likely to allow it, some places do because there is so much traffic due to the place itself, there just isn't anywhere else nearby to park. The places that charge admission mostly allow parking because of security issues with the loads, but that would be in major cities
The other thing they do to discourage parking is enforce local no idle policies on their lots, selectively, for the most part.