The air suspension on my Endeavor takes forever to dump (if it dumps at all --- the rear wouldn't). I removed the air dump valves on it and cleaned them. Now it dumps in about 20 seconds. It's not hard and took me about an hour to remove, clean, and replace all three. On my Endeavor, there were two for the front and one for the rear. That seems odd to me, but that's what it had. There's a tiny orafice inside the valve that gets crud in it and you simply have to clean it out.
Just a word of caution. Regardless of what your guage shows, there may still be air in the suspension system. So, DO NOT get under the coach when removing the air lines. Try to dump all the air first and then remove the lines from above or from the side. I was able to remove the front lines (from the valve body) from above. I opened the front panel on the front of the coach and could reach the lines from there. The rear was an arms length from the right rear corner, so I didn't actually have anything under the coach but my arm. If you remove the air line nut slowly, it will start leaking as it nears the end and will leak down gently. Nonetheless, do NOT get under the unit unless you have the jacks down or some other way of supporting the coach.
Here's the link to the procedure:
Hope this is helpful.