Bladders are ok for water and fuel, but not a good choice for sewage holding...fittings have to be owner installed in most of 'em, and they're highly prone to blow out a fitting at the first sign of a clog...they're designed to hug the contents, so it's just about impossible to maintain a bladder aerobically, which just about guarantees foul odor out the vent (and btw, USCG regs prohibit unvented bladders for sewage holding, so it has to be vented) when the toilet is flushed. And those are just a few of the reasons.
So your only two good options are, enlarge the access or find another location for the tank...there are prob'ly at least two that would actually be much better than the one you've decided on...I'll be glad to help you find one.
Check the Ronco Plastic catalog...They make TOP quality thick-walled water and waste tanks for a very reasonable price and has more than 400 shapes and sizes, over 100 of which are non-rectangular (which means you might be able to find a shape that will fit through the access), and they install fittings in the sizes and locations specified by the customer when they make the tank.
Ronco Plastics Marine Catalog