I bought my trailer used and firmly believe the previous owner only added poop, and sometimes a little bit of pee, and never any water.
I have been using a cup of laundry detergent and 5 gallons of water. I add those to the empty tank after I dump at the end of a trip. During the travel home and the travel back to the next site in a week or two, it sloshes around and cleans the tank. I then use the tank during my trip (usually only 5 nights). Before I dump the tank, I completely fill it with water... completely FULL and drive over to the dumping station. I dump all that water, fill it again with another 5 gallons and laundry detergent.
I'm getting the gunk out pretty good, even got a fork out once (I have the clear elbow on the drain) and will likely stop using the laundry detergent by the end of this season.
I don't get a lot of smell, but I tell the wife and kids to put a little water on top of the gate valve after flushing, acts like the S-Trap in a residential toilet and no odor comes up from the tank.