Septic malfunction

You need to put one of those biocide digesting deodorant liquids in your tank.  Every one has his/her favorite.  But most of them work ok and will likely get rid of the odor.  

As for your tank gauge, don't believe it.  It lies.  Crud sticks to the sensors and sends false results.  Ours has been at 3/4ths full since we first got our trailer in 2016.  I tried all the tricks to clean off the sensors and none worked worth a darn.  Now, I use the zen of tank capacity to determine how full it is.  When it seems like it ought to be full or when it is really convenient to dump, I take care of it. 

+1 to Offgrid. I have noticed the same thing. The shower one is the most likely culprit. The sinks tend to get used but if the shower isn't, the trap can dry out and then there is a clear path for odors, especially if you are running the vent fan.

As for the sensors, I use a biological tank treatment. TankTech's Rx is one product. Unique Products RV Digest-It is another. Happy Camper makes one but I have not tried that one. The principle is to seed the tank with bacteria that will break down solids and break down the odor causing compounds instead of just masking them with a strong perfume like some non-biological products do. The side effect is that it can help with keeping the sensors working (gray tank) or helping so that when the black tank is emptied and flushed, the sensors may start reading again. That is, until it is used the next time and things slosh around as you are going down the road.