Honest questions are never silly -- Start with the fresh water tank about half full. Dilute the required amount of bleach (~1/4 cup per 15 gallon capacity) in at least a gallon of water and pour it into the fresh water tank inlet on the outside of your trailer using a funnel and plastic tubing. You could also use a funnel inserted into the end of a filler like these offered at Camping World, Water Tank Fillers - Product - Camping World
Make sure you use the fresh tank filler, not the city water hookup. Finish filling the fresh tank with water, and then use your pump to distribute it throughout your lines. Let it set for a few hours, dump, rinse, dump, and rinse again. Hopefully that will do it.