Yep, I second the mention of many terrible, bumpy roads in California, sigh. By towing a trailer we Californians get major experience in preparing for the inevitable "Big One" (earthquake). I use every trick in the book to keep things from flying in my trailer when under way. Bungee cords are my best friends!
As to the porta-pottie...Our Dometic brand came with the new Scamp
and works well. It is a basic two-part system with the upper tank holding the flushing water with chemicals and the bottom holding the "ahem". We use it for "number one" only (except in case of emergency) and put used toilet paper in our small closed top trash can which we empty every day. Needless to say, when emptying liquid is easier to pour without paper or other solids.
I have lined the under bunk cabinet floor with a carpet remnant and have an area rug in the area outside the cabinet. We store the porta-pottie facing inward (the handles face outward) so we can easily remove and replace it by sliding without fear of scratching. For convenience at night, the last one to bed, sets out the porta-pottie on the floor and puts the TP on top of the lower bunk (where no one is sleeping) and the trash can next to it. While traveling the porta-pottie stays in place in the closed cabinet.
I have made a simple TP holder by removing the core and placing the roll in an empty, but clean container (Appropriately, one in which prunes are sold. LOL) I cut a hole in the top and, voila, an inexpensive and useful TP holder!
Before you decide on a port-pottie you might want to check out the owner's manuals to see how they operate. The Scamp
website has the Dometic owner's manual posted under the "Ask Scamp" tab. Look in the right-hand column for "Owner's Manuals".