Yeah I think it’s fair enough we got that one wrong. The OP said the book was about the tanks... and Cemetery world is decidedly not about two tanks. It’s also true that SF paperbacks often use art created for other books and which doesn’t represent the book they cover. My edition of Philip Farmer’s Dayworld has a painting of the urban monads of Silverberg’s The World Inside as the cover, but which the publisher ended up not needing for Silverberg’s book, so they recycled it. It’s quite common.
Inr! Now that i am reading the synopsis I am remembering more and more of the book and its rich and varied characters. For some reason those two tanks just stuck in my memory. But thank you for the big help in finding the book and title. I've ordered myself a copy from eBay