You are correct 40 gallons per tank.
After you take delivery and the warranty is registered call Thor and ask for the waster water diagram. That diagrams will show you which fixtures are plumbed into each tank and give you an indication of how often you may need to dump.
On my 37ND for instance everything on the passenger side (shower, outdoor kitchen and washer rough in) are plumbed into the rear grey tank while the kitchen and bathroom sinks are plumbed into the front grey tank. The one toilet is the only thing plumbed into my black tank.
US Army (Ret)
2020 Entegra Accolade 37TS
2019 Jeep Grand Cherokee Trailhawk (Toad)
FMCA - F432054