So just as a matter of law, whether or not a mortgage is a "disguised mortgage" does not depend on whether the asset is personal or real property. It matters whether the asset is a "dwelling" which mobile homes are. And other facts and circumstances.
In my opinion, rent credit differs from a disguised mortgage because the rent credit can be used to purchase any asset in the portfolio not the home in particular that is being lived in. How often does someone rent house X1 for 5 years and then decide to buy that different house Y2 when they've saved enough? I don't know about Frank's (or Sun Communities') experience, but that happens actually quite often in our experience (we do not do a rent-credit program though). In my opinion, the defense of the rent credit program to a regulator not being a mortgage, is, "which house is being mortgaged? -- we don't know until the end!"
This is just my speculation and not legal advice for which I have done zero research. Of course none of this has been tested in court and Dodd-Frank is enforced by the CFPB with no money from Congress who left it to the states to enforce. So, whatever your state regulator says is the best you can go by right now.