I'd guess 10 gallons, but the first time the DEF guage drops to 3/4s, try putting a 2.5 gallon container of DEF into it. Ours will take almost exactly 2.5 gallons to fill at that point so I've surmised that ours is 10 gallons. I suspect it has more to do with the chassis builder than the motor home brand. In our case, we're riding on a Spartan.
Jay & Peggy Monroe with Dolly
Can't take it with you - don't plan on leaving any behind
2016 Newmar London Aire 4553, Spartan chassis
2017 GMC Acadia toad