Here is my story so far. I did contact Heartland, worked with a guy that contacted the design group and they replied that the earlier models were plumbed that way, but the newer ones had been re-designed to fix this issue. I wasn't able to get more info from Heartland, but I noticed that the people on the forum that had the problem showed 2020 model years on their profile, so I assumed (mistake on my part) that it was a first year issue.
Yesterday I got up early and drove 6 1/2 hours to the dealership, the closest one with a 2414. I started doing a walkthrough with the sales guy, and noticed that even thought is was a 2021 model the manufacture date was 03-2020, so it was already a year old. Red flags went up and I mentioned the holding tank issue, so the sales guy had the service manager come out. We hooked up a hose and verified that water sent down the sink came out the same valve as the toilet. Everyone at the dealership was honestly surprised, and are working at finding me a new model because this was a deal killer for me, not to mention I am not enthused about buying a "new" camper that has been sitting on the lot for almost a year. They said to give them 24 hours to find a new one that they could direct to my local Blue Dog dealer at the same price. I'm not holding my breath.
So I drove back home last night, without a camper, and this morning I called Heartland again to see if they can give me a cutoff date of when this model was "re-designed". The rep said he would have the design group send me an email to let me know, but I haven't heard anything yet from them or the dealership. The one I walked through has several issues that needed fixed, and I question the durability of the Lithiums over time. I understand they call this an "entry model", but that doesn't mean I want to throw my money down the toilet. "Entry Model" is a joke, because you would have to have some value left in a camper when it comes time to trade up, and they don't build them well enough to have any lasting value from what I have seen unless you buy a non-"entry model".