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 panel.
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 !