Since the black and gray water systems are separate, your problem is likely to be in one system, but not both. Your problem appears to be related to gray water...not black water, but I'll address both, just in case.
Hopefully you are using something like this each time you begin camping: https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1
Be sure to use enough for the tank's capacity.
Make sure your sewer vent is functioning correctly. It's likely that the black tank is the only thing venting through the roof stack, so be sure that nothing is preventing proper venting. You mentioned that air pressure down the toilet from a leaf blower forces air out of the vent, but the vent should be open all the time. On a home, a sewer vent is just a straight pipe from the sewer line to the home's roof...no baffles, caps, flaps, etc. Your vent may be blocked in some way...including intrusion by a nesting animal or bird.
Note that in a home toilet, there is a water trap. But MANY (perhaps not all) RV toilets do not have water traps. They rely on a shutter valve and seal to prevent black tank stench from coming back through the toilet. If you can isolate the stench to the toilet, I'd suspect a faulty seal on the "flush" valve. If the seal is bad, odor can not only escape through the roof vent, but it can also seep past the faulty seal. Bear in mind that black water waste may generate a moderate amount of heat, and the stinky air will rise right back out of the toilet valve if it can.
This is a separate system. It should not be connected to the black water system in any way other than possibly feeding into the same dump pipe as the black tank. BUT, IF THE GRAY WATER DUMP TIES INTO THE BLACK WATER DUMP, check the valve separating the gray water from the black water dump line. If that's not closing completely, you could have black water backing up into the gray water dump line and into the gray water holding tank. That would not only contaminate the gray tank and make the contents of the gray tank VERY unsanitary, it would also make the contents of the gray tank stink far more than normal. My PUP has separate dumps for gray and black....and they cannot mix, but many larger campers join the two together and terminate in one single dump system...relying on gray water to help clean black water out of the dump hose.
Note that gray water should NOT be hazardous. It should be nothing more than water "contaminated" with soaps and a small amount of food waste. By rights, you should be able to dump your "dishwater" on the ground just as tent campers do. If you have cross contamination between the black and gray tanks because both use a common dump pipe, it needs to be addressed.
Others mentioned dry traps and poorly functioning "cheater vents." Based on your schedule of camping trips (July 4 then September), that's plenty of time for the traps to dry out. Perhaps RV antifreeze will evaporate more slowly, but a simpler tactic is to keep an old orange juice bottle filled with water in the camper and about once a month pour a bit of water in each trap. This phenomenon is common in home "floor drains" in basements, etc. You have to pour water into the floor drain regularly, or the trap will dry out and the stench will drive you out of the house.
When I'm done with my camping trip, I first open the gray tank dump, then I use a garden hose going wide open into a sink drain to flush out the residue in the gray tank. Remember my comments about the gray water not being hazardous. I just flush this out onto my driveway and run enough water through the system (perhaps 10 gallons or so) to dilute any gray-waste residue. Food waste left in the gray tank will stink like hell in short order, but by flushing out the tank with this large amount of fresh, clean water, the tank is pretty clean, and it doesn't smell. Do this at the dump station or ASAP upon getting home, so that the particulates in the gray tank haven't dried onto the tank walls and floor. If you do this at the dump station, you may want to use your own dedicated hose, because the dump station hose will be pretty gross.
While you're at it, check all connections on your sink drains. They can have a nasty habit of rattling apart. There's a chance that some gray water has collected under the sink. If so, it will stink for a long time. If you find any loose "nuts" on the drain plumbing (J-trap, etc.), tighten them (carefully) with water pump pliers, then secure them with duct tape so they don't vibrate loose.
If you want to start 'fresh', you can sanitize both the gray tank and the black tank just as you do the fresh tank...only with a slightly stronger solution of chlorine bleach. Close your dump valves. Using the advertised tank sizes, dump chlorine bleach down the drain and down the toilet in about twice the concentration required to sanitize the fresh tank and plumbing. Use the garden hose to add water until the tanks are full. Let it sit for a day or two to enable the chlorine to kill all bacteria. Dump both at a dump station.
Unlike your fresh tank, you won't need to flush the chlorine out of the tanks with clear water, because you don't drink the gray or black water. Chlorine residue will remain in the tanks for at least one use cycle.
I can dump into my septic at home. The above scenario is not a good idea if you dump into a septic. I do NOT use bleach or anti-bacterial products in RV waste water (or in our laundry), because it will kill the biological action so crucial to proper functioning of the septic tank. Dump stations can handle the bleach because they either are connected to municipal sewers or they are holding tanks that are routinely pumped out and hauled to a municipal sewer system for dumping.
Last suggestion: food particulates in gray water can be minimized resulting if FAR less stinky gray water. We always use our paper napkins (paper towels) to wipe off as much food residue as possible into the trash. This dry wiping is pretty aggressive, to ensure that virtually no solids go down the drain. We even dry wipe egg yolk off the plates as much as possible. The same is true of cooking grease, olive oil (in marinades, etc.) and so on. We do our level best to keep food out of the drain and in the trash instead. The less food the less smell.