Just wanted to jump in here. Make sure you fill your old black tank with water. Mine looked great, and the original pan was almost complete, but when I filled it with water, it leaked like a sieve. It was broken at the Thetford flange. I could have gotten the original glass on flange from VTS, but then would have to use the Thetford valve. Also, would need a new pan, and then I still wouldn't have any good way to add a gray tank. $175 is the cheapest I can get a galvanized pan made for around here (called 4 shops) Then fiberglass, Thetford setup was about $75, and then I'm left with a pretty small black tank that is about 50 years old, with 50 year old technology. Plus, since I got a new toilet, I have to cut out the old female thread ABS, and move it back an inch and re-glass a new one on.
Here's the better option that I came up with:
Sheet of 4x8 ABS was $100.00. I got 2. That's enough for 2 grey tanks as well as my new black tank. I can slope the bottom of the black just a wee bit to help it drain to the new Valterra in back. The gray line gets cut where it used to go into the valve just after the Black valve, and instead routs across the back of the trailer just inside the access panel. Now, it drops to the street side of the black, and runs up to the front of the trailer where it enters the gray tank. That one empties into the one behind it, then that follows back to the back, where it goes under the gray "supply" line wraps the back of the black, and joins the Valterra mixing valve, and ends at the same spot in the back as the original hookup. I strugged for a year to figure out how I wanted to add this in without pulling my tub out.
Since my floor covering is up, I'm driling new elevator bolt locations. (snapped off the old rusted ones, poked them, and pulled them out from the top) Dropping new bolts down for the support straps, and then drilling some wood screws in to the flange I'm putting on the black tank.
The new black tank still has to have that odd cutout for rear, floor mounted tanks, so I couldn't use any tanks off the shelf. Going to put in a tank heater, but I may just run my original furnace duct line back there with a gap in the insulation to allow it to surround the tank like the original.