I think we all know how tricky it is empty the flush tank during this time of year to prevent freezing between trips. This valve is not only easy to open but extends the outlet just enough to stop the last dregs trickling down inside the compartment.
Basically this is a catheter valve available via ebay at http://m.ebay.co.uk/itm/Flexicare-Ez-Flow-Catheter-Tap-Valve-/222064025145?nav=SEARCH but almost made to measure for the job for £3.30.
Got this idea from the ASOF that deserve the credit for a great idea for vans fitted with the Thetford C200 toilets.
You have to put the neoprene pipe that comes with it on the nozzle end and cut about 1cm off the tapered end. I also had to put some plumbers ptfe tape on the nozzle to get a good tight fit.