Any insulation that is installed under the floor is going to make comfort increase in a travel trailer. I've done three, so far. Keeping water lines, drains, and tanks from freezing is a tangent from insulating the under belly since you need some sort of heat to keep things from freezing (assuming parked or travel at a low temp). I'd suggest to consider some heat tapes or tank heaters.
FWIW, I have a tank/drain project for my trailer scheduled. I plan to enclose the single waste tank (maybe add a small grey tank) and all the drains right out to the drain exits. I will use 3/4 extruded polystyrene clad with aluminum sheets to enclose the areas. Heat will be supplied by a 60' length of gutter cable and a heat tape toward the outboard ends of the drain plumbing. All of this nonsense is inside a steel frame. The GFCI circuit with switched receptacle is already installed. It will allow me to camp at any temperature and be able to dump waste. A side benefit is that all of this is directly under my bathroom area and should serve to keep that floor warmer.
