The
Heartlandrvs.com website lists gray capacity as 90 gallons. That would be split across Gray #1 and Gray #2 at 45 gallons each.
The washer, shower, bathroom lav typically drain into Gray #1. The galley typically drains into Gray #2. But looking at the 3760EL floor plan, the washer appears to be lower than the bedroom/bath, and mid-ships. It could be tricky to run the drain into Gray #1, so it's possible it could drain into Gray #2.
One way to tell for sure would be to dump both gray tanks. Then run 2 or 3 cycles on the washer. Open one tank valve at a time to see which tank has water in it.