Cazbaz, the main problem that I can see is the structure of the water tanks compared to the structure of the grey water tank. To me the water tank on our van is a thinner plastic (1.5mm) than a GW Tank (5mm) that I installed on a friends campervan and also does not have straight vertical sides, this makes it harder to connect fittings into the tank, say 40mm or 50mm inlet/outlets plastic fittings. ie the inlet up high and the outlet low.

I would use valves to control the GW into the GW tank and to stop GW going to the usual outlet with another valve to release GW from the GW tank. These hand operated valves can be purchased at an irrigation store. ( total of 3) However the position of the valves could be hard to reach under the van and electrical operated valves may be the way to go, at the time the only electrical valves I could comes across were 24v so gave them a miss.

The 40 or 50mm piping changes are not involved ( size depending on your current piping), just similar to storm water 75/100mm piping as per DIY in SA with a bit of glue.

With the value of your caravan, I suggest you purchase a ready made GW tank to fit your existing tank hangers say from Atlas Tanks 1800666125 or www atlastanks.com.au where the unit for the campervan was purchased.

PS if getting a unit built definitely get a high bung hole that you can remove the bung to put a hose in to wash out the tank. and do not purchased small valves try for the 40 or 50mm type to me 35mm would be the minimum, and make sure the GW outlet fitting is as low as possible.

To me you are looking at more free camping in the future

All the best

Peter