Okay. I fully drained the black tank and flushed through the toilet for a good hour until I was getting completely clear, clean water. Then I topped the tank off with fresh water and dropped the coroplast.
Bingo, the black tank has a major crack on the front corner. It is clearly very old as the tank has a build up of sludge. My suspicion is that when they loaded the trailer on the lift it was tilted down in front enough to have all the contents rush forward and out the crack.
My problem therefor has nothing to do with my fabricator. Interestingly, a little over a year ago we were on a trip and we were getting a terrible odor that I couldn't seem to control. I suspected that a rodent had somehow gotten into the sub-floor, died and was rotting. I guess this answers that question. I never let the tank fill completely, so, unless the trailer is in a radical nose down position, it probably seldom overflowed and maybe only slopped out the crack occasionally while driving.
So, now the problem comes in replacing the tank and replacing and patching the coroplast. Luckily the tank is clearly labeled with manufacturer and model number.
Here's some pictures:
I just got off the phone with the tank manufacturer, they referred me to their distributor and the tank is $285 and a 3 - 4 weeks out. That didn't include shipping. Next I tried a local RV repair facility I have used and they don't like to quote jobs over the phone for, for obvious reasons, but he said he would have to order the tank just like me and that their labor would probably start at $500 and go to $1,000 or more depending on what they need to do. Yadda, yadda.
Finally, the RV repair guy told me of a local plastic welder so I called him and he thought it might be fixable. They charge $85 and hour. I need to pull the tank out and sanitize it and he will do a "test weld" to see.
I think that is my plan. I will first see if it can be repaired, if not then I will order a replacement myself and install it myself.