If it is the tank to fitting joint that has been torn loose, and not the fitting itself (which could be easier to repair), then you could try West Systems gflex epoxy. I did this to repair a polyethylene kayak and it worked, but it will take some luck. You need to flame treat the polyethylene surface first to get decent adhesion. See attached for adhesion data. Also, any cracks need to be stop drilled first.
https://www.westsystem.com/specialty-epoxies/gflex-epoxy-adhesion-data/
You could also try carefully drilling out a nice clean hole and using a rubber expansion plug to seal it. Another long shot but it might work. You don't need the drain to be there, you can use the pump suction fitting to drain the tank. Something like this:
https://www.envirodesignproducts.com/products/cherne-1-1-2-sure-grip-plug?variant=12330564518006
What year is your rPod and is it an East Coast or West Coast version? The earlier Eastern ones were pretty low to the ground. If you have one of those I'd suggest getting a riser kit for the trailer so you don't have the problem again.