OT rant about keeping '77 B200 camper on the road; then a fuel filler question
That's it! Bill, the filler neck just slips in the doughnut, but that may put you having to get some steel pipe and clamping your filler hose onto that, and then slipping the pipe into the doughnut, as the doughnut has to have a steel pipe to slip into it. make sure you measure the doughnut to get the correct size OD so it will seal properly.
OR, Red Cote tank sealer also works with plastic tanks if you want to repair the original.