Trying to re read your post and having a hard time understanding what you are specifically asking. First, before anything, do the deals make sense? This would be very important to know (utilities, any poh, rents, occupancy etc).
Next , are you saying that the total sales price is 2,500,000 which is compromised of him taking back 1,750,000 and the bank note is something you are thinking about getting from the bank as your down payment? And I don't understand , are you trying to put this only on two of the parks? Do you have the 750k or is this something you are planning on obtaining from a lender?
The questions that start to come into play, are you doing this loan with this seller non recourse or personally guaranteeing the loans? Sound like they want personal guarantees but maybe he is wanting more.
Not sure if you can do this, but maybe you can pull life insurance policies naming him as the beneficiary ? If you are starting to look at that stuff though man this better be really something. Im not sure of the limitations , but you can also look into key man insurance but not sure if that is applicable to your insurance.
Give some more clarity onto the specifics of the situation for better feedback .