After a lot of research and discussion I have decided an integral water tank was the way to go.
I have been working on this on an off for a couple of weeks in between holidays.
The first job was to cut an inspection hole and create a flange. I have made quite a large one to make repainting a bit nicer when the time comes.
I plasma'd some holes to weld the flange bar from on top. A much neater way of keeping a nice edge with the lid on.
This is what it looks like for inside with the seam sealant applied
Once this was done I put a first coat of blacking on accept where I was going to weld the front wall in. Last time it will be that easy to paint!
Next I drilled the holes for the water sensor gage and outlet. I set the level gage about 20mm above the outlet so we will have a little in reserve even if it gets to show 0 on the readout.
5mm Plate welded outside
With that done it was time to bolt down the hatch and test for leaks by blowing up the tank with compressed air to 4psi.
Nice neat edge :-)
This is the pressure gage and inflation valve
Some soapy water revealed a few porous bits which were soon fixed with a bit of weld. This test was done before blacking was applied to the internal weld which would probably have solved it anyway.
I have quite a large leak in part of the tank which is going to need some more welding but its right at the top so in all probability won't cause a problem, but I am going to fix it anyway.
Next came time to call in some help. My wife had to get in the tank to hold the outlet nuts on the inside of the tank while I did the final tightening.
All thats left to do now is sort out the leak in the top then put a couple more coats of blacking on.