I live in hot FL. If you leave raw sewage in the tank for a few days between dumping/flushing, it will produce a very foul odor! Unlike a septic tank, this is not an anaerobic environment. Folks up North have less issue most of the year due to the milder temperatures.
I've tried many different ones on the market. One works better than all the rest. Thetford Aqua-Kem.
*Deodorizer - Formulated to work in any weather condition, Aqua-Kem is packing the most powerful odor control available on the market.
*Waste liquefier - Clogging is no longer an issue, as Aqua-Kem breaks down solid materials, liquifying waste and toilet tissue fast and efficient.
*Detergent - The third component of the 3-in-1 formula is the detergent, which cleans the tank walls and drain lines, maintaining the tank and lines in top shape.
*SAFETY: The 100% biodegradable waste holding tank treatment is safe even for septic tanks.
*FUNCTIONALITY: Formulated as a 3-in-1 product, the formula holds a powerful deodorant, a waste liquefier, and detergent, eliminating odors, breaking down waste and cleaning the tank.
I've been using this product since the late 80s. I've dumped my tank into our septic tank on many occasions. Zero issues. No stink from the RV. Try anything else first if you wish. Just keep this one in mind if they don't work for you.
$21 for a gallon on Amazon.
https://amzn.to/2Hc8foF