Max life on LP gas and carbon monoxide detectors is about 7 years from date of manufacture, which is often a year or two before installation. The only fix is replacement.
Stinky black and/or gray tanks happen, and the fix requires a thorough clean out. It is possible that the vent has become clogged, forcing sewer gas up into to toilet or sink. There is also a gulp valve next to the sink if the drain to the gray tank is long enough. Its purpose is to make sure the trap under the sink doesn't get sucked dry when you drain the sink, and if it isn't working, you'll get stink either from the sink or from the gulp valve.
Good luck.