Once I had the coupler fitting everything could continue again: -
The front drain with the straight coupler fitted (and glued)
I then cut a short length of the pipe (about 4" iirc), just long enough to fit into the coupler, and into the end of the Tee connector, to connect the two together.
I glued the piece of pipe into the coupler first, and gave it time to (mostly) set.
I then test-fitted the rest of the assembly again to ensure that I would get the angle of the Tee fitting correct before glueing it. I used a piece of blue masking tape to cross the join between the coupler and tee piece when at the right angle, and then split the tape, leaving both halves in place, so I could easily re-align when glueing.
I then glued and fitted the Tee piece onto the short pipe, butting it up against the coupler. (remember to ensure that you have the Tee fitting the right way around, so that the internal curve from the side of the Tee flows towards the outlet, rather than back towards the front tanks).
Remember to hold the assembly together while it sets, to ensure that the tee and coupler don't move apart while setting.
I then test-assembled the old re-used valve assembly, with the spigot-flange and slip-hub-flange to ensure that I could get the flange at a suitable angle to ensure that the valve would not interfere with the slideout.
The original galley drain valve, test fitted with the 3" slip-hub and spigot flange fittings (the spigot fitting will fit into the tee piece, the slip hub will accept the end of the straight pipe)
I test-fitted the spigot-flange side of the valve assembly into the side of the Tee, and marked the angle of the flange to ensure good clearance.
I then removed the spigot-flange connector from the valve, and glued and fitted into the side of the Tee fitting, and then glued and fitted the spigot-to-bayonet fitting into the outlet of the Tee
The Tee connected mounted and glued (to a short length of pipe fitting into the straight coupler), with the bayonet fitting glued into the end, and the 3" spigot-flange connector.
Note the deliberate angle of the flange fitting, to ensure that the re-used valve does not interfere with the slide-out.
I used this opportunity, while the glue set harder, to install the cable-release assembly.
I made my own bracket out of some scrap drilled steel strap that I had left over (rather than using the one they supplied with is designed to bolt to an RV side-wall), and I bolted it to the side of my sewer storage mod (using one of the existing bolts). You can see it to the left of the picture below, with the existing front-tank drain valve handles in the foreground.
The cable release for the new Valterra valve.
I do still need to paint the bracket to stop it rusting.
I did have to trim a few inches of the cable, which is VERY tough to cut through.
I would've used my Dremel, but I'd accidentally left the cut-off disc mandrel at home in the garage, but I eventually managed to cut through it with some tough wire cutters (which took a beating from it). Read the instructions supplied with the kit carefully ! (you need to slide the inner cable out of the outer sheeth when cutting the outer sheeth).
I secured the cable to the underbelly, using a couple of existing screws and some of those Mounting cable ties (Lowe's item # 292686)
The cable attached to the new valve
Once the existing assembly had set sufficiently I re-fitting the re-used valve to the now glued flange, and re-measured and cut the pipe to length (from the inside of the hub-flange fitting to the inside of the elbow fitting.
I test-fitted those parts, and made sure that I had alignment marks so that I could have the flange at the correct angle to match the angle of the other flange and valve, and also put the Pipe Hangers over the pipe, though this could've been done later. (I had first straightened the pipe hanger straps, since they normally have a 90deg twist in the strap)
I removed the elbow from the cable-release valve assembly (which is a little tricky to disassemble and re-assemble, due to the way that the cable assembly clips over the flange fittings), and glued the hub-flange fitting to one end of the pipe, and the elbow to the other end (being extra careful to ensure the angles were correct as previously noted).
Once those two fittings were adequately set (I allowed 30 mins before handling), I re-assembled the entire thing, getting the elbow flange into the cable-release valve first, while supporting the other end, before then bolting the flange/valve assemblies together.
The final completed installation. The silver bands are the 3" pipe hangers
I worked out where to bolt the hanger straps to the chassis, being sure to avoid interfering with the slide-out mechanism, and cut the straps to length with the dremel (which I now had with me).
I then drilled new 1/4" holes in the straps, and up through the chassis frame, and secured them with 1/4"-20 x 2.5" bolts, going in from the bottom, with flange washers, and then nylock nuts on the top side.
I fill-tested it 2 days later (having driven to the dealership and back, about 50 miles round trip, over rough road-works, for unrelated work).
I filled the kitchen/galley tank by using a garden hose into the kitchen sink.
I then opened the new cable-release valve letting the water fill the new pipe, but with the other valve closed. Thus putting the pressure of a full gray tank onto the new pipe, and the fittings as far as the re-used valve. No Leaks !
I then opened the other valve, but left the cap on the bayonet connector. Again, no leaks !
I then drained the kitchen tank slowly, and periodically checked for leaks. All was good.
I hope this post is helpful to someone, and inspires others to do some mods that help with getting rid of those annoyances/inconveniences.