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Your new tank has to fit the brackets that held the old. Well, it doesn't *have* to, but problem is that the labor becomes cost-degenerative if they have to start doing chassis work in order to accommodate different brackets. For your comparison purposes, RV service labor is a consistent $120 per hour everywhere in my metro area right now (Houston Texas). RV servicers will often post a labor advisory sign right on the front door to let people come to terms with that sticker shock before they even walk into the facility.
Can you see a tank plate anywhere on the tank? If so, it should have the manufacturer's name and a catalog number on it, and if you are lucky, it will state exact length and diameter as well. Armed with that information, you might be in a better position to start tracking down replacement options.
If you get REALLY lucky, the original manufacturer will be able to email you tank schematics that you will be able to compare to other potential schematics. What you DON'T want to have happen is to acquire a tank that appears like it should be OK, take it in for installation, only to be told blah blah blah, we can't use that tank because blah blah blah. Meanwhile the labor meter is ticking.
Are you able to post a pic of your tank?