I am #320 and 6'2". I also have a 25' trailer that we do long trips in with a family of 4. All of us are right at 6'. We have no particular problems. However, I'm in my mid 40's and still fairly capable. There's a difference between a big man that can do for himself, and one you have to help every step. Of course, it sounds like it would be a labor of love, which changes things.
The size would be less of a concern for me than having that much togetherness. Did you mention how long if a trip? For most, a few weeks in a trailer with your own flesh and blood can become challenging.