I ran out of time to get the Black water tank mounted today. I did get the old fitting off and capped it. I will drill the new 3" hole and install a new pipe fitting from the top when I cut the hole for the toilet (will be in a different spot near the vent pipe towards the narrow side). The cap looks ugly, but it will mount flush up against the underside and will not be seen. The tanks are not pressurized and this is the top side which will not hold liquid so all I did was find and thick piece of plastic material from a old tote bin, then I put a lot of caulk around the hole and used 4 screws to hold it. It should cure up fine.
It will be next weekend before I make any more real progress. I work all week and there is not much time after work. I'll finish caulking up and taping in the insulation I have during the week if I can fit it in.
I am disappointed in Lark (trailer manufacturer) and I plan to call them. With the side paneling off inside, we had a lot of rain Saturday and I found a few leaks that need to be sealed (so glad I had the sides off and it rained!). Not a big deal to fix, but poor quality control at Lark. If the trailer was not going to be insulated, it would have never been noticed since is is running down the inside of the exterior wall and right out the bottom on the aluminum. With the insulation and all caulked up at the bottom, it would mold in there and cause rust. It is important to be air tight on the sides for the insulation to work properly. If it is not airtight, you will have a thermal bypass on your insulation and it will not be as effective.
I'd recommend anyone doing a leak check before putting insulation in while you have the paneling off inside. The way the roof goes together, you can leak if they have not used enough sealer on the seam where the roof meets the side.