Decided to put the plumbing problems in one place. This is for two reasons, one to show what might happen so that someone else might avoid it, and also to possibly attract a better solution from someone else.
Both the water heater and the toilet came with 12mm push-on hose connectors. These are intended for use with very flexible plastic hose. They don't connect well to the type of stiff plastic hose used for the pushfit plumbing used everywhere else in the van.
Consequently, when the plumbing was first assembled with these connectors and the rest of the pipework, it leaked. There wasn't really any direct solution to this, jubilee clips were of no use with the stiff pipe either.
The solution was to make adapter lengths with flexible hose on one end, and the pushfit piping on the other, with a short length of convertor in between. Wasn't neat and was a pain to mop up the leak and be disappointed at it not working first time.
Because I used standard household taps, the pushfit plumbing was ideal, but equipment meant for mobile living is better suited to the flexible hose and in hindsight it would have been better to have used that throughout and fitted adapters for the taps, at least they would have been easier to get to when they leaked! There's pros and cons, but this was just one issues we had.
Or more appropriately, the shower tray drainage. I'm not sure there is a perfect solution to this in a van but we now have to live with what we have for a while.
Unless the van is parked perfectly level, either the waste water from the shower runs away from the drain hole and accumulates in a corner, requiring mopping up later and a session paddling in the shower, or it drains slowly, hanging round the drain hole while it runs away slightly uphill.
I can see that the ideal solution perhaps would have been to have the shower drain directly through the floor to the waste tank, but I have what I have. When it works it works well, it's just getting the van at the right angle, preferably flat but there are variations on side to side angles that work!
So if I was to do this again, I'd have a very short run from drain to waste tank, and a shower tray which was bowl-like and drained to the middle from a variety of angles.