Direct replacement? Not always
I recently replaced a Thetford Aqua Magic 3 with an Aqua Magic 4. Thetford indicated it would be a direct replacement. Not. Water line was too short. Luckily I had decided to install a gate valve at the rear of the toilet just in case the toilet developed a leak later on. I recommend this. A leaking flush valve and a cracked seat was the reason for the change out. Thetford does offer a 35$ rebate though. Americanrvcompany.com had the best price and shipping was fast. I ordered online on tuesday and it showed up at my door the next day.