We used to have heating oil in a large 1200 litre tank. This lasted maybe 4 months with modest usage, so would simply check it every few months and see how it looked. Average usage should over time be relatively predictable - i.e. colder/warmer. So we had no real issues with regulating this.
Your initial point seemed to be along the lines of how to charge guests - i think this cant be done without annoying them, you just need to work out average usage over time and buffer it into to nightly cost. - as was said earlier, guests would expect all basic bills to be included, so include them 🙂