Anyone own an Advance 640 and drained onboard fresh watertank?

[Euro330]

Been draining the onboard fresh water tank but cant see where I manually drain it to get the last water out of low level pipes.
The tank has been pumped empty. I have checked via the top access cap and its dry. But the bottom drain valve does not seem to drain any more having moved it to various positions. I am concerned that the low level drain pipes still have some water lieing in the bottom.
Anyone have experience with this?
Thanks.
G

 

[Euro330]

Update on above.
Well thanks for all your tips! lol
Anyway i stripped the push fit plumbing connections from the bottom of the tank along with the drain tap.
Only to find the moulding for the tank outlet which had a 22mm threaded female fitting in for threading the outlet pipe into was completely moulded over, no outlet hole!
Drilled it out.
Obviously a tank moulding/fitting feature fault.
Beacause it wasnt actually flushing through there was a lot of sedement in the bottom (crap).
Probably because I bought it from a private seller in south of England the water lime scale was building up (hard water,).
Flushed through a couple of times and tank is now clear as a bell.
Makes you wonder how Bailey would stand if there was frost damage to pipe work/tank and would propbably initially renage on warranty claiming owner failed to winterise properly.
Worth checking your own tanks if similar make model.