Hi
A local lad on this forum had 2 Combo's - 1 had 400l upright and the other a 400l flat tank.
Both fitted fine.
The lay flat tank just just left sufficient room to fit the hose reel between it and the rear doors.
Only problem with the flat tank was it's large footprint. If the van was parked the wrong way on a hill, the outlet pipe could suck air even although there was still a fair amount of water left in the tank. Vehicle handling was apparently better with the flat tank.
I prefered the upright tank, which we fitted to my son's Peugeot Partner - he can virtually empty his tank - no problems.
I am kitting out a Citreon Relay for myself to replace my Suzuki Van with trailer combination and have again fitted an upright 650l tank.
An upright tank gives you a lot more floor space for foldup ladders, hose reel, brushes and a couple of plastic boxes with all the odds and ends we carry around incase something goes wrong.
Spruce