Darn plastic fittings! Maybe the reducer installed in the molded toilet inlet put too much pressure on it and it cracked.
I had a polybutyl tee crack on me in my 86. It is the pressure line from the 12v
pump. I do not know what cracked it. The line runs under the floor. I had very limited room to work, and no chance to replace it before our trip. The repair will involve removing the fresh water tank to gain access to the plumbing lines running above the tank. Then replace with PEX.
So I lightly sanded the tee with sandpaper, cleaned it up with alcohol, and then mixed a little two part epoxy. Using my nitrile gloved fingers, I "rubbed" the epoxy into the crack.
The next morning no leak! Yea! That repair is still holding one year later and after two trips. The line is only pressurized when the 12v
pump is running. We more often than not have city water hook ups.
It is worth a shot in my view. It's no good cracked, and getting a replacement can be done when you return.
Nice looking old Bambi by the way.