So I couldn’t get myself to pay for the Sealand 711 and we didn’t want to spend another season with the Thetford in our '04 2720. Looking around I found this 8.5 gallon holding tank made by Icon that is designed to have a toilet direct mount and reinforced to support the weight of a person. I ordered one from Amazon ($140) to see if it might work.
It seemed plenty sturdy so I decided to continue. I found a Thetford low profile toilet on eBay new for $95. For venting I decided to use a marine through hull vent from Amazon.