bathroom built as part of addition drainage plumbing in a crawlspace (i.e. accessible). Work done in 2005. Toilet is a Kohler with an over-sized trap. it sits next to a jacuzzi tub that rarely sees action. symptoms 1) When toilet is flushed, a loud gurgling noise can be heard through the tub drain. 2) The toilet sometimes has trouble draining and backs up despite it not being, shall I say, abused. the sinks smell bad fairly often and need to be rinsed with bleach a couple times a year. Questions: a) Could the toilet's issues be a result of inappropriate incline on the drain line? b) is the gurgling in the tub after a toilet flush a sign of improper air trap setup or normal. c) does the occasional drainage problem, together with the odors indicate possible problems with the vent?