RV toilet bowls usually use a rubber seal to keep the water in the bowl. This water is essential to seal off the waste tank odors preventing them from getting into the rig.
If your seal dries out it can start to develop a slow leak draining out the water and causing a dry bowl. I’ve found when this happens or as preventive maintenance, a coating of Vaseline on the seal works wonders at keeping the seal nice and supple and leak proof. Vaseline lip therapy works great and comes in a nice handy sized tube for easy storage in the RV bathroom.
Just apply some to your rubber-gloved fingers and spread on the round rubber seal. Do it with the water source off. Some people advise that Vaseline being petroleum-based may damage the seal, that’s not been my experience but if your worried it might another choice is Plumber Grease.