Use certified (Cat-certified or equivalent) scales, when low on fuel, get off the bus, do what you need to to get it under 25,800 pounds (25,700 would be better). If I remember right, they round up to the next 100 pounds, then add 100 pounds (can't be for the driver, I've met very few bus drivers under 100 pounds). That would bring 25,700 up to a 25,900 registered weight.
If you're transporting stuff, you might want to wait until you get here and empty the bus, drain house water tank, waste tank, etc. Weighing doesn't cost much, so you could weigh it on the road, to see if you can meet the target weight.