My van is 6.35m, not huge by any means but parking is not hard with a rear view camera (around 50 squid off e-bay & an easy diy job). In supermarkets, you have a large rear overhang, use it - reverse into spaces where your stern can hang over a bit of grass or unused pavement, then you will fit in a normal parking bay. If you pick the middle bay of 3 empties you'll find no-one will park next to you. Even with a car either side my cab doors are high & far forward so will often open over the top of the bonnet of a small car.
Use as little water as possible & you will need to top up less often. Ask at garages when you get fuel. Also check out public loos when emptying your toilet cassette.
Relax & be flexible, there are always "ways & means" of doing anything if you are creative & ask nicely.
We haven't found gry water to be a roblem as much of the water we use gets drunk as tea/ coffee so goes in the loo! We can last a week before the tank needs emptying & we tend to use a campsite for one night for that. Think about what you let run, some isn't a problem, but a load of greasy dishes water with food bits in it isn't nice to leave in any public place as it attracts vermin & flies & will smell in due course.