Problem solved: I applied body molding tape to the two knee pads with the protective tape still on and clamped them together with 4 c-clamps done tight and left them for a week, so the tape would adhere thoroughly to the pads. I peeled the protective tape off and stuck the pads to the tank and pressed them on with a small roller. They haven't lifted at all.

Body shop supply store, but many automotive stores carry it.

I still have some 2 sided tape from when I worked at a shop. Im pretty sure its a little over 30 y/o,, BUT,, last I needed a little piece, it worked.. Todays adhesives are SO much better.. Windows in skyscrapers are held in with a tape that allows them to shrink and grow with the temp, yet not fall off...UHHMayzing..and EX Pen Sieve. I wanna say Digger posted a link to a Super Duper roll O stikkum,, I Thot I wanted some, till I saw the price..

When playing with thick rubber stuff and wanting it to stay on a compound curved surface, of course grinding the rubber to fit at least Kinda is important,, and some heat (Okay,, lets call it like it is,, Pain Compliance)
Now,, the tank,, Hoo,, boy,, Talk about wanting ALL the wax gone,, How did you get the tank surface prepared>? Ive heard Dawn dish soap is just the bomb for ripping all the oils and waxes off a car, and Not recommended for washing a car, at all..