We leave our tanks closed and dump them when needed. I don't like "sewer flies". Supposedly keeping a bend in your sewer hose works as a P-trap. Personal preference and to each their own.
Anyways....
It could just be that your sink drains/p-traps need a good cleaning. If the rig has been sitting a year, the "stuff" down there might have dried onto the piping.
If you're having grey tank odors, you may want to shut the tank then pour some Dawn or Calgon down the drain. Fill the grey tank and let it sit awhile to "de-gunk" the inside of the tank. Dump the tank and see if that helps.
And have you checked your water-heater anode rod, if you have one?