I just did one in my '74 AS 31'. Yes, disconnect gas line, undo plumbing, unscrew from outside. I have heard that some have a mounting bracket under the tank but mine did not. Removal is pretty easy once you cut through the caulk and sealants. For replacement I went to camping world outlet center in Bowling Green KY. They have a scratch and dent section w/full warranty for about 1/3 the price.
Do your self a favor, before you install the new unit make sure you have your plumbing in order inside, as there is very little room to manuever on ce the new unit is in.
The other issue I had was refitting my stainless outside access door to the new unit. I did it but it took a little doing. I just could hang a white or painted access door when I have the stainless option. Dig around here and look for Overlander63, he has pictures and basic instructions for R&R listed.
It is really a pretty simple job, but plan the better part of a day to get it done.
Operation "Save Rudy" Strike Team (charter member)
Yes, I am still working on it.