Not expecting much but I do think the NFCU Flagship covers RV parks. I mean, C'Mon it is called NAVY federal... perhaps they should throw a line to boaters? It would be nice to earn more than 2% of $6k a year in gas and dockage!
I know Bank of America includes fuel at a Marina along with other fuel types like Kerosene and #2 Oil in the 3% fuel category. Dockage fees though are something else. If they accept mobile payments you can always get a US Bank Altitude Reserve. Sounds like you might want to check the coding of your merchants with the Visa MCC locator.