Knobs off
Batteries good..no bulging....not hot (Did you check water levels?)
No smell from kitchen sink......BUT
Do you have an 'air admittance valve' installed on sink drain line to grey tank after the 'p' trap? Stub sticking up with vent slots in side....they screw on.
It could be stuck open which then allows grey tank odors to come back inside the rig. Grey tanks can stink!!!
Look for one of these under sink(s).....bathroom too

As for that LP Detector.......shelf life for 'sensor' is 5-7 years.
Time for a new one.
Edited: Was typing as you posted.
Battery levels sound low........top off with distilled water.
Might want to check converter/charger output.
Sounds like batteries were overcharging and boiling off. That really stinks.