wrybread wrote:
...Remember that in your house the two systems are connected as well...
This is not true. If your house is built to code, every appliance that creates waste water is protected by a back-flow preventer or air gap. And those are NOT the same as a check-valve. Check-valves are not sanitary. An exception might be your dish-washer, depending on your local ordinances. We are required to have air gaps, but many places allow a high loop instead.
ewarnerusa wrote:
http://www.irv2.com/blog/2014/12/rv-grey-water-recycling-made-easy/
This link provides a diagram that helps visualize this kind of mod. It links to a kit you can purchase. Our old TT had a small grey tank and this mod may have been more attractive. Our current TT has 50 gallon fresh, grey, and black tanks so filling the grey tank too quickly has not been an issue.
Image from the link above.
The only way I would do this is, using the above diagram as an example, would be to remove the connection to the 3-way valve where the check valve is shown. Then you would have two independent water sources, and the twain shall never meet. Of course it would require having some gray water in your tank before using the toilet, but a couple of gallons would do it. And in a pinch you could use the bucket brigade.
