Here's what I do if using out of hours drop off.
If you are not taking the Avis fuel option, get a receipt when you fill up. My technique is to photograph the receipt (just using my phone) then leave that receipt in the car - so that when staff check the car in they see you have refuelled. I leave the receipt over the instrument panel - so that staff have to pick it up in order to see the mileage display.
Photograph the car (including instrument panel showing mileage and fuel gauge) before you put the keys in the drop.
It's very unlikely you'll have any billing dispute following out of hours return, but I find it you have plenty of photo evidence on your side, any dispute will be resolved quickly.