2'' in 3 weeks is a lot. 1'' could evaporate in 3 weeks. Could be a combo of this and seepage? I've seen every video (they're out there keep looking) on cleaning the seal, lubricating it -- had a special emory board type wand I used. Brillo type pad, cleanser, stiff bristled brush. Allowing the drain peddle to snap back.
A 2016 is too new to have a dirt problem.
Couple weeks ago, at the Factory, my CS bedroom toilet lost water within couple days. My Tech cleaned it with something I can't remember the name of, then lubed it with something I've also forgotten. I wasn't paying attention.
Whatever he did, it fixed the leak.
Years ago, on a 5th wheel, I resolved an ongoing problem of losing water over a long period of time, I replaced the base of the toilet -- ball, flange -- and that solved the leak.
If the Tech thinks it's in the normal range, that's about right.
2017 Newmar Canyon Star 3710
2002 Class C Lazy Daze (26' MB)