It's not uncommon to come across parks where all of the tenants pay cash. It's just a fact of life when doing business in our space with a seller like the one you have. What seems to matter most in these situations is your mix between tenant owned and park owned. You have massive leverage over the tenant owned. No pay, No stay when you take over really works and you rarely lose these people regardless of their payment history with the previous landlord. On the park owned, this can be all over the map. Based on what you've said, I am willing to wager that you will probably be ok here.
When you take over, adopt a no cash policy. No exceptions. It's not hard and people almost always comply without an issue. If someone can't adapt then just purge them out of the park. FYI, out of the 800+ lots we own, no one's ever left because getting a money order was too much of a problem for them.
If your seller pushes back against holding the paper; take what they give you in your first request for documents, send it to at least three financial institutions, cc him on all correspondence, and have the bank cc their responses back to you to him as well. Hopefully, they'll bombard you with requests he knows he can't fulfill. He'll realize very quickly that financing isn't possible because his books aren't up to par. Then, give him an out by financing you for just enough time to create all of the financial requirements a bank will need (4-5yrs).