There are a lot of detailed threads on here about this very issue.
A lot of us have gotten scangauges, which is what I've done. It has not ever failed me. Unless my gen is running at full-tilt for several hours, even its consumption makes next to no impact on the scangauge's reading. For instance, coming back from Mackinac Island, last week, my scangauge said we had 52 gallons used and I ended up filling up with a total of 53 and some change.
They are a bit pricey, but not having much of a clue as to how much fuel you've used, how much you have is certainly worth it!
'14 Jayco Seneca 37fs
20' Look Enclosed Trailer