There is no access cover, but I am not sure you have to drop the drive shaft, exhaust and rear bumper. You should just have to lower the front of the rear cradle to access the 2 bolts that hold the tank straps. What year is your CTS?
Loosen rear cradle bolts and remove front cradle bolts and let cradle hang in front all the way down watching right rear corner of cradle for a harness that may get pinched if not careful. The strap bolts are still not that easy but are accessible...some what. Hope this helps but looking at the date you may have repaired it by now