Welcome to the forum. I also own a 31 BHDS and you will find that this model will give you a lot of room. As far as the wand going into the tank, you should be able to hit the bottom of the tank with it. There might be some clog in the down pipe which would keep you from hitting bottom. Most of the wand will be in the tank. When you turn on the water, you will hear the spinning spray hitting the bottom of the tank. At that point, I lift mine up off of the bottom of the tank.
Recommend you have a clear plastic connection on the sewer hose to verify when contents have emptied.
2004 F150 SuperCrew Lariat, 5.4L, Max Towing, CIPA Towing Mirrows, Timbren
2006 Jay Flight, 31 BHDS
Reese Strait-Line Dual Cam