Sometimes you have to really look at it, and imagine from all angles. Bottom looking up is easy. How much room above the frame , can I get a long box wrench in there? There's a hole in the bottom angle, if I slip a wrench in , drop small end out the hole , then grab the nut on the inside? Oh , take tailgate off I can squeeze in from top gap?
Or , if I drop the , tank, muffler, tailpipe, whatever , an inch or two, I can get it. Longer wrenches, fingers that bend backwards, with one eye closed, small kids hands with gorilla strengths.
Yeah , you try to avoid the " off n on " parts to get your goal completed, but sometimes it's unavoidable.
Look at what ya got work with , and how can , from all angles, views , can I accomplish this.