Check out this thread
If you have all of the pieces, you may be able to reassemble it. I rebuilt mine on a '73 Sovereign
using Evercoat fiber reinforced epoxy (bondo)for similar reasons and it is working fine. Getting to it from the inside is a lot easier if the coach is gutted (mine was at the time).
You'll have to disconnect the filler hose and remove the housing from the inside of the exterior wall (it is firmly attached with vulkem and olympic rivets on the outside) and care must be exercised to avoid further breaking. (It is very brittle.)
Once it is repaired, re-installation should be done with some sort of backing washers on the rivets to avoid cracking the housing (again, it is brittle).
Another option (assuming you can make the door stay on) is to insert a long flexible hose from the outside and fill the tank using a funnel.