I don't get the drama about a flat tank being a problem. I just park the van 'the right way' so the outlet is at the lowest point.
You can’t necessarily do that though can you especially in a one way street etc if you live in a relatively flat area like Lincolnshire you would prob get away with it but you certainly can’t ware we live , I don’t know a single cleaner that has a flat tank as they won’t work here .
My area has some flat areas and some pretty hilly bits. My outlet is on the nearside front so generally works with the camber of the road.
But what happens if you are facing up a reasonably steep hill ? I found it was drawing air as the water went to the back of the tank once you get to half a tank full and the outlet was above the water level
Very occasionally you have to be mindful which way round you park the van, making sure the water is angled to the outlet, but for me, having a 750l tank, that’s rarely an issue. I can make 750l of pure last almost all week! So on the day when it’s only got say 200 litters left, as it runs low, I just have to be a bit more thoughtful on how I park. It’s very rarely an issue.
You also have the option to carry 2 or three 25l drums of water to top up the tank if necessary.
I will probably be going to smaller tank when I get the e custom, so I’ve been toying with the idea of having a 100lotre small upright tank in the van as a backup. I’d use this if and when the water in the main tank gets very low. It’d be permanently plumbed into the same hose line to the pump, but with its own isolation tap. So if only turn it on when needed.
There are so many options, it’s pretty easy to navigate round these small hurdles.