Haven't had to look into this yet. I would start with NAPA, then move to an Airstream Dealer/RV dealer. Then to RV salvage yards, then to marine or RV reair shops. Worst case you could get a tank fabricated based on the original dimensions, mounting tabs, etc. Perhaps even have baffles added to keep that gauge more accurate.
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Steven Webster
1986 Airstream 345 Classic Motorhome
AIR 1760
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