Don't have a Frontier but tow our 400 with a Titan and it tows great.  No need for WDH at all.  The Frontier should have plenty of hp and torque to tow a 400.  You'll need to watch your tongue weight and sag.  I'm not sure what your max tongue weight is but I'm sure you would be fine.  Our 2021 400 has a tongue weight of somewhere from 380-400lbs and a loaded weight, with empty water tanks, of about 3100lbs.  I did add some Road Active Suspension (RAS) and sumo springs to our Titan since I have a camper/topper on it that weighs about 400lbs.  It does an amazing job with eliminating sag.  If you have any sag or want to carry more in the bed I would recommend it.  Takes just a few minutes to install and rides great when loaded or unloaded.   I'm not a fan of WDH as they can put tremendous strain on the trailer and vehicle frames under certain conditions.   Your truck may be fine with some sumo springs and or some RAS.  

EUREKA!  I figured it out.  I found the below photograph of the area below the shower and toilet floor of a 2015 T@B Max S.  It was posted by @Matt in 2017.  I added the labels.  It is a rare view of what that area looks like, at least for earlier model T@Bs.

As depicted in the photograph, the previously unidentified hole is where the shower water ultimately drains to the gray water tank.  So, my confusion was in not realizing that the shower floor drain, which connects to a P-Trap, is connected to a black PVC pipe that empties into the gray water tank (red arrow.)  Thus, the shower requires TWO holes through the floor.

Mystery solved, and good to know info for future reference.  Thank you working through this with me.

att