I did the very same installation
with the same Thetford Nano, but without a y valve.
The deck wash pump is rated at 60 psi and has its own pressure shutoff switch (runs only on demand).
I checked with Thetford before I installed this system.
The schematic diagram I mentioned seemed over their heads, but they said it sounded like it would work
I used a diode to isolate the toilet inlet solenoid from the washdown pump switch on the electrical
When the washdown pump is used for upping anchor
, it works.
When the toilet is flushed, the washdown pump gets its power through a 12 volt relay which is triggered through the diode which gets its power from the toilet solenoid.
The toilet can flush water
a lot faster than the bigger washdown pump can supply it.
Ees bery seemple !