What year is the van?
My 2007 500/5 has the usual drain tap just in front and slightly left of the water heater. Can upload a full photo if needed.
Have you just acquired the van?
Ah righty. Cascade heater I think, so valve external on heater.
Look for a round plastic screw about 20mm diameter with a slot across it. There's no need to remove it completely, just loosen it.
Michael will probably be the one to confirm this, he'll drop by sometime soon enough.
This will be on the outside of the van, it's a section of the heater which protrudes thru the bodywork.
Look on outside wall immediately adjacent to where heater is located, on offside towards the back of the van?
we have the 500/5 2003 a different heater though.remove shower head and place in the sink as these can freeze at great expense.pull shower pipe out as far as it will go and leave it hanging.leave taps open at half way point and cover with an old thick sock.drain the toilet flush tank from the pipe just over the toilet cassette outside,a small pipe with plug.remove ANY books,papers or towels that a little mouse can make a nest.remove ALL liquids including drinks cans as these can freeze and make a real mess.remove toilet fluids as this can freeze,well mine did.clean fridge with antibacterial liquid spray and leave the door wide open,also clean wall over cooker and sink,splashes unseen can mould.after cleaning leave the internal toilet cassette flap open to prevent it sticking.after draining block the waste pipes outside to stop bugs getting in.fit plugs into ALL drains inside to prevent smells from the pipes.if you cannot remove cushions to the house stand them on there end in the centre of the van.remove shower curtain and keep folded in a plastic bag as these can mould/mold.wipe vegetable oil on the toilet seal to stop it drying out.cover heater with some old blankets to prevent freezing of any bits of water.half way through doing my jobs now before putting on the new cover.
------------- the only silly question is the one you do not ask.