Black plastic tank ?

I was having a look see under the offside of our AS Northants and found a black plastic tank which has a screw cap on the bottom.
Presumably this is a water tank, or maybe not. It is situated close to the LPG locker and almost directly below the interior fresh water tank situated under the offside bed/seat.
There is no filling point or pipe/hose for filling this tank, if it is meant for fresh water storage. The waste water tank is at the other end of the van at the rear.
Perhaps a second fresh water not connectd up?.

Best wishes for a Happy New Year and safe travels to us all.

Eddie.

A photo and size would help.
Could it be the expansion tank for engine coolant.  This is only a wild guess

Thanks Safariboy and Peter for your informaion.
A friend has an AS Boughton and as you say Peter the water system on his is as you say, with the exception that an internal tank pump is used as opposed to the one fitted inside our van under the wardrobe
Thanks again.
Eddie

What's the purpose of the two tanks, is it just to give a greater capacity and why one inside and one outside?

Hi Paulmold.
Your guess is as good or better than mine. Probably as you say, to increase the amount of freshwater that would be available.

Could it be to reduce weight transfer during cornering/braking? It's just a guess, but the fact that they use small bore tubing between the two would make that theory a bit more likely...

A Bourton has two tanks as, apparently, there is not enough room under the floor on the Mercedes chassis to get a larger tank in. The Mercedes is rear wheel drive and the prop shaft gets in the way so I was told. A Bourton only has a fresh water capacity of about 60 litres and, I guess, the tank inside is about 20 litres so about 40 underneath.

I haven't crawled under the van to see what room is underneath so am only relaying what I was told at the NEC when ordering the van and enquiring about a larger tank.