I don't think there's an overnight fix. Here's what I'd suggest. It's worked for us:
Gray tank:
Do not put any food items down your sink drain. Take the small extra step of using wet wipes to remove food from dishes, then throw away with trash. NEVER put grease down your drain... you're just asking for a nasty stink! Think of the sink washing as a sanitizing step. That way only soapy water goes into your gray tank.
Black tank:
Starting with a WELL FLUSHED tank, put about a cup of any liquid laundry detergent + about 1/2 cup of liquid Calgon. Any detergent will do, and any type softener ("slickener") works... Calgon was always our choice, but is getting harder to find. You DON'T need expensive proprietary stuff unless it makes you feel better.
Then (IMPORTANT!!) add about 5 gallons of water to the tank. Yes it takes away from tank capacity, but it's a necessary evil if you want to avoid stank.
As you drive the detergent will slosh and coat the interior of the tank. As you add poop to the tank, it will mix with the detergent solution.
When it comes time to dump the black tank, drain it completely. Then fill it at least 3/4 full with water. Dump. Repeat.
Then add the detergent/softener and 5 gallons of water as before to start all over again. Your gray tank doesn't need any special treatment IF you get all the rotted food remnants flushed out.
These steps DO take a little extra time and effort... anything worthwhile always does. But your tank "stank" will be a thing of the past!