YOu really need to neutralize the residual salts left in the bore from both Black Powder, and the substitutes. Even Pyrodex leaves salts that combine with moisture to form acids, and in very short time.
Use a cleaning patch, with spit, if nothing else is on hand, to swab the bore. That will remove most of the salts- with a couple of patches.
I have cleaned my barrel in the woods with just a cleaning patch and spit, and the gun stayed clean enough until the next morning when I was home and could clean it properly. I didn't find any rust, but I was hunting on a very cold, dry day, too, and the gun didn't even go into my car for the trip home until it was in a case. The only moisture in the car came from me, and that was not enough to rust anything. The gun was laying down on the bottom of the hatchback, below where any "steam" might have reached.
If you have oil, or alcohol, but no soap and water, then at least coat the bore with the oil, or use the alcohol to flush out both salts and carbon deposits.[You can get soap and water at most gas stations. So, only long trips out of wilderness areas on horseback are going to cause real delays to cleaning the gun.] The oil will keep AIR, and moisture, away from the metal, and crud. The alcohol will dissolve any oils, loosen and dissolve both salts and carbon and flush them from the barrel.