To safely prepare your 100 gallon RV fresh water tanks after contamination, first carefully remove the dead mouse using gloves and dispose of it properly. Next, thoroughly clean the tank using warm water and a mild detergent to remove any debris or residue, rinsing completely afterward. Follow this by sanitizing the tank with a food-safe solution such as a mixture of water and vinegar or a commercial sanitizer, then rinse thoroughly again. Be sure to inspect and clean your second tank as well to ensure it is free of contaminants. If the tanks have unsealed outlets, consider sealing them to prevent future contamination. These steps will help ensure your fresh water tanks are safe and ready for use.