A couple of thoughts. Not sure why a tankless would fill the grey tank any quicker. You have an existing hole in the side of the camper that is the size of a 6 gallon Atwood. (the recent 4 gallon and 6 gallon are the same size, I presume the '83 was the same but I have been known to be wrong
) A new Atwood 6 gallon would fit that hole exactly I think. How would you mount / install a tankless? Are you able to custom fabricate something to fit the existing hole? Is there a real advantage to a tankless heater? Price difference?
My inclination would be to put in a 6 gallon Atwood. .... but that is just me, if we were all the same we would all own the same model camper.