What is up with "Park Model" homes?

They are a much higher risk. Lenders will look at them with about the same enthusiasm as they do single-wide mobile homes. Most often they are on rented ground, with all sorts of things built onto them. Things like decks, skirting, patios and the like. Often the wheels and axles are removed. All those things make them impossible to repossess. On top of that, to take one off of a rented site, they will have to pay all the back site rental. We once had a division that financed mobile homes. Our general policy was if the loan failed, it was best to just write off the loan, send the people the title and a 1099 for the amount that was written off. Much cheaper than hiring a company to go and move the mobile home and then find someone who could sell it for anything more than a few thousand dollars. Most often the cost of moving exceeded the value. You can sell a trailer to anyone since they can drive in, hook it up and drive it home. A park model, no way.
I am sure there is a lender out there that thinks they can re-invent the wheel who will finance them, but it won't be a line most lenders will get into.