Your black water & fuel vent is easily clogged by spiders, and all types of bugs and toilet paper. You must clean the outside of every vent line with a small brush at least every Spring. Your black water vent is very important, it may also be cleaned with pressure water from the outside, or also use an air horn. Tank watch – remove sending unit regularly and clean well. Make certain that monitor cycles thru empty, low, mid and full ranges.