I feel for you...I'm just finishing up the exchange of the 6 gal unit in our Ambassador!
Our rig has soldered copper tubing fresh water plumbing...really a pain to deal with, also AS used 5/8 and 5/16 somewhat oddball sizes when they plumbed these older rigs...
I'm sure AS originally installed the water heater before mounting the cabinets around it...making the replacement without tearing out all the cabinets, a much 'tighter' job, to say the least!
I went back with a poly hot/cold 5/8 tubing to the heater on both the cold and hot sides...I adapted to the existing copper tubing using 5/8 compression type fittings...after giving up on trying to reuse the original plumbing at the water heater.
I went back in with an Attwater 6 gal heater (like the original) with the pizo ignition system...it was easy to mount the switch in the cabinet above the heater, using the 12 volt
supply to the water pump next to the heater.
I'd previously installed a new variable speed 12 volt
water pump...an improvement over the original.
After all my bruised fingers and sore knees recover ,I'm sure I'll fell much better about this 'upgrade...right now I'm resting my sole with a nice toddy while taking a brief rest...