Hmmm...
You may try a box or open end wrench. It will be slow, but will save you from having to loosen the tank, which would likely end up with unacceptable "project creep"!
If you cannot get any wrench into the space, then you will have to loosen the bolts holding the tank straps and attempt to slide the tank from side to side. The problem here is, if the tank has not been moved recently, it is likely stuck to the padding with rust. If that happens, you would have to remove the straps and rock the tank to free it...and pray no leaks develop as the rust is broken loose.
I would try every wrench I could think of to NOT have to move the tank!