We did this with first our Bounder, and now our Country Coach.
On the Bounder, due to tank and venting access, we spliced into the Grey at about 1/3 of the way from the bottom of the tank. Then fed into the black at about as close to the top as we could get.
On the Country Coach, same with the Grey about 1/3 of the way from the bottom, and then the black was fed into the Blacks venting pipe.
Both used the OEM Shure water pump, with a newer improved water pump going into 'fresh' water pumping duty.
We also added an inline filter screen between the Grey and the pumps (Grey to Black now.) inlet side. I added the cleaning of that filter to our regular maintenance schedule.
One thing that was interesting to find out, was that both I and the place that sold me the Country Coach from consignment (A Southern California CC dealer.) - was that after we completed the installation. The tech said 'Lot's of work for a 40 gallon black.'... When I negotiated the price on the coach, it included 100 hours of shop work at the dealers base rate (Break even vs the then $125 shop rate. It was either $70 or $75 for labor. Plus I got the parts a price, plus 5% to cover dealer handling of the parts. We knew we would be making mod's, so baked that into the deal. Took the Salesman two days to beat the Service Manager (Who inside of the Dealer, took the budget hit for the discounts. Vs the Sales Department
!) into agreement...). The Salesman, an stand up honest gent (They do exist!) and the Service Manger, and myself - all thought we had a 60 gallon Black. We called then just reopening CC to ask, and the tracked down our coach number and confirmed the Tech was correct, only 40 gallon.
We still find that while boon docking, that the ability to move Grey to Black, extends our days off the grid by two more days.
I feel for those that do stay off the grid for longer periods, that this is a worthwhile mod.
Best to all,