I just went through the same thing and purchased a park that was in foreclosure. Same situation: little to no records. It's a challenging, but not impossible situation. The easiest, and most effective, step will be to personally go collect the rents. The most important thing is to verify that the income the seller says is being generated by the park is actually there. Beyond that, you can look up tax, get insurance quotes, etc. and essentially assemble your own financial picture moving forward. (Verifiable rent receipts -- along with any leases -- should go a long way towards addressing a bank's major concerns.)
The big challenge will be getting a sense of the shape of the park -- both what you can see and what you can't see. I'm guessing there are little to no maintenance records, so you'll have to spend a couple of extra bucks to have a plumber and electrician and septic tank guy, etc. come out and help you do the due diligence on the park. Again, it's challenging...but you can compile a pretty good picture of what you're getting into by being meticulous and going the extra step. Good luck.