I did the same,.. went wfp when I had a pug partner, but bought the biggest tank I could physically squeeze into it and then changed for a bigger van as soon as I could!
Jonah,... 300 - 400 litres a day will be more than enough for an average day,.. BUT don't plan for an AVERAGE day,.. plan for the longest busiest day you can imagine,.... you can never have too much water, but its very easy to have too little.
I'd recommend a 650 litre tank for a 1 man setup if your van will take it. I have a 400 litre tank and I do find it too small on busy days.
If you're in a soft water area, topping up DI only from a custies tap might be an option (With their permission!), and a smaller tank might be an option then,.. but for the extra few ££££ a bigger tank is a sensible option.