A couple of things I see conflated in all this stuff.
Most states offer some kind of tax break or advantage for a "homestead," that generally means your primary residence, that you spend most of your time in or intend to return to. Broadly speaking, you cannot have two homesteads, certainly not within one state. The ramifications of this are state law penalties, like you owe back whatever tax or other advantage you obtained by falsely claiming a second, or third, homestead.
That differs, at least in legal effect, from representations that might be made to a mortgage company or guarantor like FNMA or VA. The definition of "principal residence" and "vacation home" and all that differs from state designations of homestead. They may be similar, but they're not the same and not prescribed by law, but rather by the rules of the mortgage company or guarantor.
The penalty for making false representations to a mortgage company/guarantor comes in potential criminal liability for mortgage or bank or wire fraud. And possibly foreclosure.
An incorrect or even fraudulent homestead designation is not mortgage fraud. First of all, it's not a representation to a mortgage company, but rather to a state or state taxing entity. Second, the definitional differences can be significant. For example, you can have more than one "principal residence," but not more than one homestead, typically.
Homestead designations can be, perhaps, EVIDENCE of mortgage fraud. That is, to the extent the definition of a homestead under state law is the same or overlaps that of "principal residence" or whatever representation you make to a mortgage company, your homestead claims may undercut or belie that or vice versa.
It seems the richie riches and political types commonly claim more than one homestead. They deserve whatever penalties state law calls for.
But that isn't mortgage fraud.