How do you know when it's actually full?

[BarkingCrow]

2019 High Country 331RL - 49 gallon black tank.

The reading on our gauge will show the black tank is completely full after 2 days, which is obviously incorrect. There are only 2 of us, so a tank this size is not going to go from 'completely empty' to 'full' in 48 hours. It's never been a concern, as we have always been at a park with full connections, so I'd just dump the tank after a week or so. But now we're parked at the farm where we'll be building our house. We don't have a septic tank yet, so I'll have to take it over to the RV park to dump the tank. I'd rather not have to deal with packing up the loose stuff, connecting, etc., once a week, much less paying the dump fee this frequently, as I highly doubt it is necessary.

So... how do I know when it's actually full, since the gauge is pretty much useless? Any tips?

Thanks,
D