Rolf, I think you have a good idea.
Some other things to consider:
In your letter to home owners in other parks you could offer a referral to those who send a homeowner dirt tenant to your park.
You could offer to carry the note, after a $500 down, on the move and make some extra money on your money.
Or, you could simply offer a new big screen flat panel (about $1200) for new resident homeowners. That could be a big draw for you.
I saw Costco had a pair, one large and one small, flat panel Magnavox HDTV for $1200 at the Costco website. I think a small in the bedroom, large in family room would be a fantastic incentive. Especially if you promoted it for a move with the income tax return check.
I’d pull together a check list about the schools in your area, the features of country living and city access. and how they would outweigh the extra time getting to work/gas. (don’t I remember your park is about 20 miles outside the city?)
I think there are lots of incentives you could use to get people to invest that tax return in a move to your area.
Not a pro here, just thinking out loud.