5th Wheel RVs have a basement. This is the area behind and below the king-pin where you'll find a massive storage compartment. This area is so large on many new 5ers that you could use it for separate living quarters for your mother-in-law.

One of the problems I experienced with my 5ers was access to the stuff in the center of the basement. One of the ways you can solve this problem is to install a cargo tray that slides your gear out for easier access. Another way you can access you center basement is to install a trap door over the top.

Here is an example of how a fellow RVer used this technique to have easy access to a freezer. He installed an electric outlet in his basement and put in a freezer. He cut a hole in the floor, reinforcing the the area around the hole and installed a trap door.

Now they can easily access the additional freezer and a few items in the center of the basement. I would not do this without first consulting with the RV manufacturer because there may be structural problems (cross beam supports) that may prevent you from doing this. It never ceases to amaze me at how creative RVers are when it comes to modifying their rigs to suit their purposes. Keeping you up to speed on cool ideas - Jim Twamley, Professor of RVing