I know it seems a bit backwards, but I fitted my water tank last in my water system. I also installed a couple of float switches into the tank and wired them up to two indicator lights so I could easily see when I was starting to get low on water.
The next stage of my water system was to plumb my kitchen tap as well as the grey water tank from from my sink. This would have been a very simple job if I wasn't such a cheapskate and bought and incredibly cheap tap that leaked everywhere...
The next part of the water system was to install a shower unit in the rear of the van. These types of shower units are intended to be used outdoors, in well ventilated areas, as they are gas powered. I have mine mounted right at the rear of the van and will only ever use it with the doors wide open.
I started off my water system by installing a Shurflo water pump and a Fiamma accumulator, along with a simple On/Off switch as I don't need the pump to be on unless I'm using my tap or shower.
As you need to get your gas system signed off by an engineer, I thought I might as well pay for them to install it too. I opted for an underslung LPG tank that feeds directly into my hob and shower unit.