As for just combining the grounds, it seems like it would work? But I haven't found any definitive answer why GM did it, best I can figure (and this comes from a comment on either one of the threads I found or one of the ones you mentioned) that it could be due to the Cadillac having a digital fuel gauge, so perhaps they isolated the circuit for some reason because of it being digital? Again, not sure, but I think I've nearly got it all sorted out, I just need to order some stuff. Since I have this all laid out on my bench anyways, I'll probably take pics of the mods I make to the Racetronix harness so anyone can do it on their Fleetwood in the future. The in tank is easy to fix, the parts stores all sell replacement in-tank harness that are just all the individual wires and the connectors in separate pieces and you put together what you need from it. I'll rob the 1 wire I need from it (fortunately it's cheap and it's just the ground for the fuel gauge) and add that to the in-tank harness from Racetronix.
And also yes, I haven't gotten so much as a peep out of Racetronix. I'm also not impressed by them. But I really thought this would be a simple install, just follow the detailed write-up that's already on here. FYI, as of 2017 they still list their kit as coming with a Walbro pump, but it still comes with the Turbine Tech pump that they switched to some years ago. I thought I'd read somewhere that they switched back to the Walbro after customer complaints and issues with the other pump, but apparently not. Also, if you have any insight into this Turbine Tech thing, I'm all ears. I haven't found any confirmed reports of the pumps failing yet, and what they describe is that it's a turbine pump rather than a gear pump which reduces noise but provides the same flow rates, pressures, etc, and supposedly has a decent service life.
I was going to respond to this post last night but it was late. However, the last 2 posts you made kinda read my mind as far as what I was thinking then. It's been a while and can't remember for sure, but if you look at the 3rd picture in my thread here you will see the Racetronix Bulkhead fitting and Wiring Connector.
Racetronix Kit Install
The plug itself could have a wire added and might be all you need. What I'm not 100% sure about is if the bulkhead connector has a 4th metal pin that runs from the outside to the inside where the intank harness plugs in. I cant see that in my picture. But again, getting back to the Whole "Ground is Ground" comment, you should be able to just splice thos 2 together...I think. If you have any concerns, just use some test leads to wire things up temporarily outside of the tank to at least "electrically" test that all things work as they should before committing to splicing the 2 wires.
Another thought I had was I wonder if Gary (innovativewiring.com) would be interested in taking over where Raetronix has and make/offer similar kits? I may just shoot him a PM and see what he thinks. I know the quality would be impeccable and the Customer service would be the same and waaaaay better than the other guys.
Speaking of the "other guys" or Racetronix. I've defended them for quite a while on their customer service even though I was not a believer in how they provide that to their users. I've usually gotten answers to my questions and even offered them input on things I discovered that weren't in line with theirs cause I felt it was a better way to do it. Simple statement to describe racetronix...... "They have Awesome Products and are top quality parts but their Tech Support is not the best or easily accessed when needed."
There is a reason why I've changed my tune regarding their tech Support. But unless you have some Aptitude to "back yard engineer" a perfectly good part, you may run into issues. If you do have issues and can't figure it out yourself, you could be stuck with Really nice parts that you don't know how to install.
On the turbine tech pumps, both I and another member here shared with them concerns we had with the Turbine Tech Purple Pump they offer now. The other member even had his pump literally MELT and needed to be replaced. The thing that really pissed me off was that THEY removed both mine and his posts/Questions/Concerns/Pictures of said damage from their tech forum. We supplied good info with pictures and had a very good back and forth with them on the Purple Pump. This could be helpful to those looking at info but may make their "new" Turbine tech Pumps" look bad to someone. So what did they do days later with all that info? The Sneaky SOB's deleted all that info from their website. My only assumption is they did it to conceal the fact that these pumps have issues. I responded to them to ask where that info was and the little shyts had no response and I chose to move on. But that there is some downright sneaky garbage and they can't be trusted IMHHO based on that simple fact alone.
So good products as far as the Harness stuff but don't like how they "seem" to be covering up issues with their Fuel Pump. Knock on wood mine is still working but when it fails, I will get the Walbro pump instead but continue to use their Harness.