sandpiper 5th wheel black tank venting issues
2000 sandpiper 30rlss. Our black tank develops a powerful vacuum about once a year. It does not have a flush connection. If I open the valve to drain the tank,or sometjmes even with the drain valve closed,the toilet will not flush. I can see what was in the toilet sitting in the drain pipe below the ball in the toilet. There is no obstruction on the vent. I can run water down the vent with a hose and it will run through the black tank out the outlet and the toilet or the material in the pipe below the toilet will still not go down into the black tank. I have one if the flush attachments for a hose. The only way I can remedy this run the attachment on a hose through the drain pipe into the black tank and move it up and down with the water on full force. During this,the vacuum is so strong on the tank it will blow what's in the tank back into the toilet bowl. I know it has to be a vent issue because I can hear air being sucked through the ball on the toilet while this is going on. Eventually running the flush attachment up and down will remove whatever is blocking the vent and it will work. I only have one vent through the roof,so i'm assuming the tanks are vented together.I removed both vacuum breakers from the sink traps but no help. The gray tank never gives a problem.....ideas?