These comments are based on my own experience of living on the road since June 2017.
There are few facilities in the UK for motorhomes that are not commercial camping sites.
Many petrol stations have an external water tap. Spend a few pounds and they’ll usually let you fill up. Keep a look out for water troughs and drinking fountains. Many churchyards have water taps.
It depends what type of toilet you have and if you can avoid using it. Keep in mind that most supermarkets, shopping centres and many garages have toilets for public use.
If you have a wet toilet cassette, empty it in a public toilet. If you’re near a canal sanitary station use that, buy a key from the Canal and River Trust.
Avoid using the blue variety of chemicals. If your waste ends up in a septic tank the chemicals destroy the bugs that allow the tank to break down the waste. I use bio clothes washing pods in the cassette.
Thick varieties of toilet paper can clump in the cassette. The cheaper thin brands break down quicker and reduce the likelihood. Some people don’t put the paper in the cassette preferring to burn it.
Caution: Wet/baby wipes do not break down and will cause blockages. Ask anyone who clears drains for a living.
If you use a composting toilet use cat litter, sawdust/shavings or something organic and absorbent. Do what you need to do in a composting bag throw in some litter etc. When it’s full tie it off so it doesn’t spill and put in the rubbish. It will breakdown over time and become compost.
Most of what we throw away is packaging from the supermarket. If necessary, I leave rubbish in a bin liner in a supermarket bin. Some councils put bins in lay bys. Sometimes one is fortunate enough to meet up with a “bin lorry” on its rounds or find a large commercial wheelie bin.