Remember your black tank is nothing but a hold tank. There are chemicals to help fight the smell and help things break down but most are only good for about a week. We get these little packets at Walmart that are already proportioned
http://www.walmart.com/ip/Porta-Pak-Portion-Control-RV-Marine-Holding-Tank-Deodorizer-Packets/16783721 and just drop a packet in after I flush tanks with a gallon or so of water in the tank. Don't want dry tanks as they will stink more. If we go longer than the week sometime I drop an extra packet in and it seems to help. There are different versions of Porta-Paks and we use the orange.
The other point is dealing with any gasses. Keeping a little water in the bowl will help keep the seals sealed and from drying out. Since the gas has to go somewhere it should go up your vent to the roof. Ensure your vent line is clear. You can remove the cover on the roof and run a garden hose down it without water unsure its not clogged. Wasps and other insects build nests in them and clog the vents. Once you know its clear you can run some water down it to ensure its cleaned out. There are new vent covers which are designed to help vent the tank outdoors like
https://360productsnorthamerica.com/site/. Flushing the toilet with the vent fan on will suck gasses from the tank into the camper.
I have added water softener (not fabric softener) to the tanks when traveling. It seems to help loosen the stuff that wants to stick and stink up the tanks. Newer RV's appear to come with a black tank flush system which really helps flush the nasty bits from the tank. A clear elbow
http://www.walmart.com/ip/Camco-RhinoFLEX-90-Degree-RV-Sewer-Hose-Swivel-Fitting/23636867 between the RV sewer connection and sewer hose will let you see what's coming out. They come in straight, 45 and 90 degree elbows. I prefer the 45 degree one. I flush my black tanks until I get clean water and no chunks. Also waiting until you have 2/3 or 3/4 full tank will unsure a good flow to move the nasty bits out of the tank.
If you flush black tank(s) before the grey tank(s) the gray water will help rinse your sewer hose.
Hope that helps.