Well, we finally had a chance to take a close look at our issue today. Interesting find. There was a small pile of TP just below the toilet hole, but not a mountain blocking it. After seeing what the BT flush didn't get, I'm thinking forget that, just drop the back ramp, bring a hose in, and flush the tank from the toilet itself - which is what we did this evening.
Here is the real issue/culprit. The hole Forest River cut in the BT doesn't fully line up with the straight down drop pipe of the toilet (I was wrong about thinking it was slanted). There is a crescent moon shaped section of the BT hole that obstructs a part of the bottom of the toilet pipe hole. I'm guessing that when something other than a "pee" is flushed, particularly if enough water is not used in the flush, sh*t gets hung up on that crescent moon causing a clog (pun intended).
And this misaligned hole issue is the cause of our leak in the underbelly. When I only filled the BT to just below the crescent shaped misalignment of holes (doing so with a hose in the toilet with a flashlight), no leaks from the underbelly. As soon as I tried to fill up the BT so that the water rose up into the toilet drop pipe - no dice, couldn't get it to fill beyond a certain point and it began leaking from the underbelly.
So, obviously, once water gets above the misaligned holes, it begins running on top of the outside of the BT, and leaks through the coroplast underbelly where pipes come out around the gray spray foam insulation. Query - shouldn't there be some type of seal or sealant where the toilet pipe dumps into the BT????
Also, and this scares me a bit, when crawling around looking at the underbelly, I noticed that our coroplast underbelly has bowed down under the BT and is actually touching the TH's axle - presumably from the weight of the water that went on top of and outside the BT instead of inside it.
I'll be calling Couch's tomorrow about this. Since we're about 2 hours away from a reputable "bricks and mortar" FR rep (refuse to use Camping World that's about an hour away as they have a horrible service reputation), I have our mobile RV repairman that we've established a relationship with coming for a service call the week after next. I haven't filled him in yet on our discovery tonight, but will do so first thing tomorrow. Anyone have any thoughts on how tough a fix this could be? Think it could be something that could be done "on site" or does it sound like something that needs to be done in a shop? Wondering whether the BT will need to be dropped. Any thoughts/advice are much appreciated!