Originally Posted by Sirwalshalot
There is a picture of where the tanks are and there is a "thing" that is part of the plumping and I have no idea what it is.
Thanks in advance
That "thing" looks like an "air admittance valve," which works like a vent but is sort of a "poor man's" solution to your overall venting issues. You really should keep the separate vents for your galley area and black tank. From your drawings it looks like you plan on just one vent over near the shower and bathroom area, and that's a pretty long run from the kitchen sink without a vent.
I replaced my three tanks last summer, and I had a bad experience having them made. I did use the black tank that I was having made, and it matched the original exactly. I ended up buying an Inca fresh water tank (found a good match) and then an Inca black water tank that I used for a gray water tank. I had to get creative in positioning, securing, and sloping them but was able to use the original galvanized belly pans that support and protect them, and now have a gray water tank that hold about 24 gallons. I also utilized the three vents that were originally there.
There's a lot of info on these forums about tanks. I think you will find that unless you have your tanks custom-made you will have to buy "close matches" and then get creative. There are many ways to do it, and many good threads here.