Our 1st mod to the RV was to maximize our storage in the cabinet above the TV by adding a shelf; it's now much more efficient for us now.
We changed out the bathroom faucet to a more usable and nicer high necked one and added a hand towel holder. The bathroom looks much nicer now.
To top the bathroom off, my absolute favorite modification of all is the high powered shower I got for Christmas. The problem here is that I don't want to get out of the shower. 5 Stars for this.
Lastly, we added a ladder to the top bunk. My son likes it because now he doesn't have to jump off the top bunk and fall to the floor.