I have only ever worked with a flat tank. Never (in 4.5 years) had any problems with airlocks, even when I have emptied the tank without noticing, I just switch the pump off, put more water in then switch it back on. I think airlock problems are more to do with a system that hasn't been thought out properly. Anyway, back to the tank, its fine and a flat tank is always better, if you have the space, as it has a lower centre of gravity and therefore makes your van more stable.