Hard packed dirt is fine,if it isn't raining. That's usually enough room for a 30-foot RV plus the vehicle towing the rig, or a vehicle being towed, and enough room for access around the back. The hook-ups should be right at the edge of the concrete, so you don't have to roll off the concrete to do the job. I agreed that grassy sites could work but ground conditions will vary. If you select a park with a grassy site, you might want to keep an eye on the weather.
Rain can quickly make the site boggy and slippery for wheelchair travelers.
We have a 26-foot travel trailer with a lift, pulled by a van with a lift. When we park, we need a wide, level spot that will handle two lifts. There is a great selection of locations where the hook-ups may be on the various recreational vehicle makes and models. The electrical and phone outlets may be in the middle with sewer and water connections at the end of the vehicle. Some of the newer units even have the hook-ups centralized.