Depending on how long you've let it set without camping, it may have dried out. Ours did the same thing when we bought it. It was used. We filled the tank up with water and a bottle of blue dawn and let it set at least over nite. Then rinse it out real good. Hope this helps.
Walmart or your local RV store has packets to drop in toilet and make sure you are rinsing when ever you have full hook ups.
As well as a good deodorizer (odoban from Home Depot), put about 2 bags (20 lbs) of ice down the toilet with about 1/2 the tank full of water. Go for a drive. The ice will act as an abrasive to knock loose anything "stuck" to the walls of the tank then melt. Empty the tank and you should be fine.
Having lived in a 5th wheel for ten years, we put toilet chemicals (can be purchased at Wal-Mart) into the tank. Also when traveling we would put a bag of ice in the tank to knock off the "stuff" that was stuck on the side. But always keep chemicals in the tank!
Pour vinegar into it and let it sit for a bit if you need to scrub it a bit use some baking soda as well this will definitely clean it out.
you can also go to 1rv2,com i was lead to this site from a KOA and its all about rv questions
When we had our motorhome, when we got ready to pull out of the parked area we would drain the black water(toilet) water, then run through the sink and shower water. After that was done then i would pour hot sudsy water down the toilet, i poured probably 6-7 dishpans in i then also added some bleach into the water i put in toilet. Then as we trasvled down the rtoad that clean waste would slosh cleaning that black water tank..i dfid this every time we would move or travel down the road. Never hasd a bad smell. We also used the toilet deodorants as they aslso broke down the solid waste.
A good cleaning after the toilet water has been drained. I don't like Clorox but this is when I use it with lots of water. Leave in about 20 minutes. It disinfects but you must empty the water and clorax after you have done and rinse with more water which will go into the drain outside or additional chemicals would make poisonous gas.
We used a blue deodorizer from an RV store after cleaning and this was used with some water. Wish they had something better for the environment than these chemicals.