There shouldn't be a big differences between coaches. Are you comparing to a previous coach or has this problem just cropped up?
If both tanks are draining slow, I would suspect a blocked or partially blocked vent pipe....bees nest?
It seems unlikely both tanks would have a drain blockage, especially the grey. If it's difficult to go up top and run a snake down the vent, then you could fill the grey tank. Make sure all traps in the coach have water in them. Then have someone in the coach when you dump. If any drains gurgle, then you have a blocked vent and will have to figure a way to clear it.
John & Clare - Buck, Paige & Zeus - 2 Jacks & Siberian Husky
99 National Dolphin 5330 F53 V10 - 5 Star Custom Tune
Towd: 2 HD Streetglides