Leaving the black tank valve open when hooked up to the onsite dump station is an invitation to disaster. There are two main issues, the first being it is GOING TO PLUG. In the deep tank of a Motorhome, it will work till it doesn’t, then it is time to call in the pros to fix it. In a bumper pull it will work for a short time, but the tank will form a pyramid and plug. In a 5th wheel, if it lasts more than a few days, it’s on borrowed time. The issue is the tank system is designed to flush 70% water and 30% solids. Varying much from that means a plug is on its way because the tanks are only a few inches deep. Trailers are easier to unplug because there is usually a straight drop from the toilet to the tank and the pyramid can be broken up more readily. The tank in the typical 5th wheel is 4 foot below the toilet with two 45 degree turns between the tank and the toilet. Add a tank that is barely 3 inches deep to begin with and a plug will form very quickly. Even with the valve shut, 5th wheels are prone to plug because they are not flushed with enough water to move the solids away from the 3 inch downspout. The other thing that is going to happen, especially in warm weather, is bazillions of little itty-bitty flies and all sorts of sewer dwelling creepy crawlies now have direct access to your coach and will enter through the toilet or drains. NEVER leave your black valve open unless of course, you enjoy the challenge of unplugging a toilet and tank.
If you are already plugged go to the link on this web site on how to unplug a tank or www.tankdr.com to see how to use a TankToolRx to unplug it.