Water pump won't prime

If you have a water pump that has problems priming, or if pump over-runs and fails when still 25% left in tank, the problem maybe the submersible water pump lying on its side at bottom of tank.
The pump needs to be vertical to work properly.
This remedy on my 2018 Warwick Duo probably applies to other vans.
With ramps under rear wheels for greater clearance, get under van and find the white electric lead for submersible pump entering centre of tank at top of forward side. Gently pull the cable until you feel the pump lift off bottom of tank. You should be able to tell when the vertically aligned pump just lifts off bottom. Place a tight cable tie on lead where it enters tank to prevent the pump falling on its side again. All done!

Thank you for your post which I found useful.  It didn't resolve my problem on this occasion for my 2016 Winchcombe, but it was a worthwhile check.
 
With the pump running and the cold shower tap open I tried sucking on the shower hose.  This brought water through but still the water did not flow when the pump was running.
 
The Autosleeper FAQ mentions trapped air around the impeller preventing the pump from producing enough pressure to open the non-return valve in the pressure adjustment unit.  A potential remedy recommended by Autosleeper is to disconnect the pipe from the pump and very briefly run the pump to clear the air from the impeller.  I found this was successful and when the pipe was reconnected and the pump switched back it generated enough pressure to open the non-return valve and water flowed through the taps.