i have a 650 tank in the back of my transit, but with continuous jet washing ive used it all in about 40 mins tops.
Its usefull for the little jobs i.e. graffiti removal.
I used to use a trailer and i hated it. for the same reason as poleman. its a pain!
I use a transit, but it is a little cramped with the tank, the machine, hoses, chemicals plus my window cleaning stuff. Mine is a SWB, so i would sujest something a bit bigger.