Being as it's a used unit you have no knowledge or how the previous owner maintained their holding tanks.
I had a trailer I bought used that had the proverbial "poop mountain" in the black tank because the owner used it as a Park Model and just left the valve open.
I poured a cup of Dawn in the tank and filled it 3/4 full with water. Took the trailer for a drive, parked it and let it sit for a day or so.
Drained the tank, filled it 3/4 full again, went for a drive, and drained again. Problem solved. No more "poo cone" and the smell went away.
Reason I only filled tank 3/4 full both times is that I wanted water to slosh around to clean the inside top of the tank too. While driving a couple of swerves when pretty much alone of the road helped slosh the contents around. Leaving it all to set overnight insured that the soapy solution would penetrate the dried dung so it would flush out easier.
For cleaning a stinky tank I'm sure there are hundreds of family recipe's but I've found that good old Dawn is a good initial cleaner for built up solids. From there just about any recipe is good as long as the tank is well flushed when emptied.
BTW, if one starts smelling a lot of urine smell it should be obvious that a good cleaning of the throne itself, inside and outside of the bowl, and offenders should take better aim.
"A wise man can change his mind. A fool never will."
"Sometimes you're the dog, sometimes you're the tree"
2018 Flagstaff Micro Lite 25BDS
2004 Nissan Titan