Best Plastics of Cassopolis, MI was bought by Meyers Industries. However, Ameri-Kart (Division of) manufactures the RV tanks, both water & waste.
Go to their http://www.Ameri-Kart.com
website or call 800-670-6804.
You may be able to find the drawing at their website - I once found my freshwater tank drawing. Check it out.