For MHP purchases, I would and do use the same contract for every state, and sometimes "my" contract is the basis for the final contract and sometimes it is not. But I'd always have a state-specific lawyer look over a financing contract because they're so specialized.
A contract is pretty important to understand how it would be enforced in whatever state you get into court in.
Remember, in general this could be "your" state, the state of the "other party," or some other state if some event concerning the contract "happened" there. For instance, say Seller retires, moves to Hawaii, and then dies (could happen) and probate is in Hawaii court and the court finds you in breach of contract and awards damages to the heirs.
This same attorney (or a similar one) is going to be the one defending your rights in court, presumably.
None of the above is specific to MHP's. If you're working out a deal (written contract) for buying or selling something, whether goods or services or land or anything else, you ought to at least talk to a lawyer if you're going to need the courts to help you enforce it (or face the courts, as the case may be) and it's going to be worth going to court over.
I would probably declare the law of the state of the real property as the governing law right there in the contract, regardless of where the seller is located (or you). It may seem obvious at the time what is meant (even if unwritten), but if you are suing the Seller, you don't want Seller to have "home state advantage" if it's possible. And you have a pretty good argument that there's no good reason Seller should object.
That said, you could probably use the same contract (with minor variance) for each state -- the point of the contract is to write down what is supposed to happen in every eventuality so that there's no ambiguity. Of course you can't predict every single outcome, but state law will "fill in the blanks."
There's two types of blanks. Those that state law (or federal law) requires and/or that you can't over-rule, and ones that you can over-rule if you and the other party agree to do so (fill in the "blank")
Any good in-state lawyer will know the "tricks & traps" IN THAT STATE, and that will be addressed (in your favor, hopefully) to write in the blanks that state law has as a "trap" for your side of the contract. You can't do anything about the ones you can't over-rule, so my philosophy is don't worry about those, these laws are usually reasonable.
Such a "trick" might be a law that says, "Late payments accrue no interest charges unless otherwise specified by the parties." You leave this "blank" if you're the buyer (and then don't worry about paying late!) but if you're the seller, you want to make sure your late payment policy is in there. So as the buyer you could also just write something in ($50 late payment fee and no interest) and expect the lawyer to flag it if it's illegal or not in your favor compared to what is standard practice in that state.
This is why there are lawyers to advise you. You cannot rely on exclusively on Frank (or anyone else, including me. I am not a lawyer in your state and this is not legal advice!)