First off, you don't dehumidify the tent. You dehumidify the room the tent is in.
Secondly, it takes time. When I first put my dehumidifier down in the basement after we bought the house, it ran non-stop for a week before it got it down to 60%. Unless you buy a VERY large dehumidifier, or you have a VERY small space, it takes a long time to get all of the moisture out of the air.
By dehumidifying the air in the room, the humidity inside the tent gradually lowers as the air is circulated through it.
If you have the dehumidifier in the tent, it's too small an area, so it runs sporadically and has no chance in hell of ever actually curing your humidity problem.