Re: Possible to install larger fresh water tank?

If you don't dry camp much I can't see why you would go to that expense, time and effort. It would certainly void your warranty, making structural changes like that. I don't know where you camp but we've never seen a campground that didn't have at least water. A lot of them don't have sewer for sure but almost all we've ever been to have water, or water and electric.
FR would have many reasons to deny a warranty claim if you modified a brand new fifth wheel in my opinion. I would think about that aspect first. Good luck and Happy Holidays to you!