I get what you mean. There are many other ways to make more money per square unit of land than an RV park, be it just a passive solar farm, or renting the land out to a nearby farmer so they can have more range for their cows.
The problem is that RV parks are already on a razor's edge when it comes to profit. When states and areas start demanding retrofits/permits/code changes, etc., it can push a lot of parks over the edge, so they become empty lots that banks own.
RV parks are also vulnerable to recessions. Hotels may be in use due to work/business related items, but an RV park either gets people on vacations or people staying for the season. When times get tight, there are few people who will be staying.
Even at $40 a night, it is a bargain compared to hotel fare which is easily upwards of $120 a night, although the bedbugs are a free gift.
