The 5er had two cross members, one fixed and the other was bolted each end to the frame. If the hole is in the one that is bolted, you can support the tank and remove that cross member L bar shelf, so now you can get at the hole in the tank.
If the hole is in the fixed shelf cross piece, you could remove the other cross shelf, and shift/lower the tank off the fixed shelf so you can get at the hole. Huge PITA
ISTR the shelves were for the higher part of the tank that stuck out so the upper overhangs would sit on the shelves. Once the water is down below that it should be past the hole. The tank will stay at least half full if that is the case there.