Since you have a plastic surge tank you have a FL chassis. There have been numerous Newmar owners who I know who after 5 or more years have had their tanks start to leak. You can get a steel tank from FL but will be a special order and cost more of course. That's the way to go.
Tom/Teri 16031 NKK 117995 FMCA
18 London Aire 4531 Spartan K-3 605
97-04 LA 04-18 Essex 2018 London Aire