Alan,
I might have a trailer coming up for sale. Its got a water tank built into it, and two compartments (one front and one back)
It's a trafalgar trailer that used to have a hot water machine in it. Will come complete with all the hoses run etc if you wanted to plumb your machine into it. Tank is 600L
Trailer is an IFOR Williams. I'll be wanting around a grand for it. Its in good nick, tyres are all like new, no rust, hitch is great, tows very well.
Similar to this, except there is no stickers, its all Galvanised metal. Hinges and locks work good and all gas struts work well.
Its a bit bigger than the picture and the slopey bit at the front is shaped a bit different but you get the picture.