An aftermarket warranty is not, on average, going to be a winning proposition for you. Warranty companies do not stay in business by paying out more (again, on average) than they charge, but rather the opposite. Of course, that doesn't mean that any particular situation will necessarily come out that way.
I think the best protection against large unexpected expenses is to do a thorough inspection before purchase, both of the chassis and the house parts. This often might involve hiring some third party to check things out.
At least in the case of the Ford gas chassis, and I suspect also for most diesel motorhomes (though I'm not really up on their details as much), the mechanical vehicle parts are quite reliable if given appropriate basic maintenance. Problems with the house things are more prevalent.
(You might also want to investigate class C's. A lot of class C's are more set up with families in mind rather than couples, and a lot of class A's the opposite. There are, of course, plenty of exceptions on both sides.)