It's a black tank. It doesn't smell like roses to begin with. you can let it sit for a couple years as long as it didn't dry out and have no issues. If it did dry out, you just fill it with water, let it soak a few days then drive with ice cubes in it when you take it to dump. It's a holding tank or a storage tank, whatever. I bought a used TT that was owned by some snowbirds and sat for 8 months. It came complete with a wonderful filled black tank. I dumped it and went on with life.
The person who mentioned a bit of oil in the toilet to stop evaporation had a good idea. Wile it is true that the flush valve will keep the toilet sealed if it runs dry, the rubber seal can dry out and will need replacement.