Good question... I've never done that,, but if I did I would think a semi soft rubber mat,,, just a bit of cushin under the tank,,, but won't hold moisture, as in if the tank sweats.. Nice looking rv you've got by the way !!! resembles ours a little...
Monkey, pilot of a Great Dane hauler,
2015 Silverado 2500 Duramax/Alison 4x4 CrewCab 2016 Cougar 28SGS