A lot of new construction boats are using poly or built in FRP fuel, water and black/gray tanks.
If you can get a correctly designed poly, it would be on lighter side like aluminum. The newest designs do expand in hot weather, contract in cooler weather. Some older designs will expand, not contract back.
Look up portable marine tanks to farther understand issues I'm talking about. Attwood has good info on their website. Where I work, we have portable transfer tanks to 50 gals for marine use. They have wheels, hose, nozzle, and need a foot pump, or small 12v battery, like a U1 size, to operate.
I've been using poly portable1-5 gal tanks for 30 years with no issues. I have no use in the past, or current future use. I would consider a poly before a steel. No aluminum tanks to my knowledge.
Marty
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