finally got a reply from INCA plastics and they not longer make any of the original tanks for my 77 31' sovereign
I started my search for alternate options for tanks that will allow us to keep a similar to factory plumbing layout, and so far I have found that with some slight modification the Fresh water tank for 70-75 Airstream will work so I ordered that from Vintage Trailer. I am waiting for their generic 55"X21"X4" Grey tank to come back in stock to order. My issue still is the black tank. I would like to keep the dump connection in the same location. the old tank which is no longer available is this https://highskyrvparts.com/new-airst...ay-tank-106659
I found a similarly shaped tank that is smaller that i can still get but the capacity is down roughly 6 gallons which may be an issue since we plan on going on some longer trips as our family of 4
If I am reading correctly the manual for my year trailer the Fresh water tank is ~50 gallons, Auxiliary(grey) is ~13 gallons and the black tank is listed at ~23 gallons
Any guidance or help on a tank that would work for me for the black tank would be greatly appreciated.