First I would break off a jig saw blade shorter so as to not cut the tank and cut an access hole around that sewer line big enough to have the freedom to do the repair. I would make the access cover later, perhaps a split in two pieces to go around the pipe for future access.
Or box it out with a compartment door leaving the very bottom outside but the two valves to be inside
I would get all new ABS fittings and valves and not use the environmental glue, it's sort of yellow in color, use the real stuff that's black for abs instead.
Now when you talk about the poly tank to ABS connection... this is a problem. Making sure the tank threads are very clean, I'm not saying do it like me, but, I would use a very good caulk/ sealant on the very threads of the male Abs before turning into the tank threads. West Marine actually has a glue suited for these poly tanks, but, how well it works, I do not know. The poly surface is a problem for most sealants.
I know the manufacturers just simply use a good caulking 5200 or some other polyeurthane. Vulkem is another form of polyeurathane and so are alot of others.
Best of luck on the repair.