I’d mix 1/4 cup of bleach with 15 gallons of water, dump it in your toilet, and let it drain into the black tank. This should also help kill any mold spores that may have formed in the piping or black tank.
And since it sounds like this happened over winter, there be moisture issues in your RV while it’s in storage. Next time you store it over winter, place some Damp Rid throughout your RV to help absorb excess moisture.