[JRTJH]

Looking at your floorplan, I'd suspect the low point drains are on the curb side, just aft of the rear wheel/axle. That would put them in line with the exterior shower and directly under the bathroom. I'd guess they are outside the frame rail, either "in the roof of the wheel well or just aft of it.

Look for a "glob of spray foam" in that area. You may need to trim some of the foam away to expose the PEX tubes. They probably will have "screw-on caps" rather than "turn handle valves", so they'll be "small and compact, well hidden by a 3 inch "foam glob"...