This is a simple way to protect the bath tub and outside shower plumbing from freezing. In a Trailmanor camper, the bathroom vent is at floor level and mounted on the side wall of the tub (between the toilet and the tub). The vent pulls air out of the bathroom and blows it into the cavity below the tub. The air then exits through a black plastic box on the bottom of the camper (the box is screened on the bottom). That same box also contains the P-trap for the tub.

When temps are freezing or below, turning on the bathroom vent fan will help to prevent freezing of the tub plumbing as well as the plumbing for the outside shower and the tub's P-trap. It is a win-win situation!

This is my Amazon Associate Link to the Reflectix material I use for insulating the low point drains and the gray water plumbing.
https://amzn.to/3W5tnld