Personally I think it is best to not cover the tanks. The cover can trap moisture which contributes to rust.
We had a soft side cover on the twin tanks on our
Scamp. It had an elastic cord at the bottom to keep it on, and a zippered cover for access the the valves.
Our
ParkLiner has a hard plastic cover that fits over both tanks. Just lifts off for access. It's a matter of looks more than function.