The 2800, so probably the 3600, has a little vent line on/near the carburetor. Make sure it hasn't been plugged with a bug nest. I wish I remembered more about that, but I know a few OP's here had issues.
If God's Your Co-Pilot Move Over, jd
2003 Jayco Escapade 31A on 2002 Ford E450 V10 4R100 218" WB