I hope you get to keep the tank. Where there's a will there's a way. And nobody's more stubborn than a reefer with a "gotta have" tank. I would fill my entire basement with concrete before I gave up that tank. I'm not sure what's under your house. Full basement, crawl space, raised slab, etc.

Cut a hole in the floor, frame it up, lay stone and rbar / grate, pour the crete, let it set, and rebuild the floor over it. I would do it, lol. That's why my hubby loves me. I'm all for demolition if I get what I want out of it.