Park the RV where the tank may be drained completely. Open the tank housing panel and wipe down the tank, then wait to see where the leak is coming from. Circle the leak with the marker. Open the tank and drain. Turn the water off if the trailer is connected to an outside water source.
Drill one small hole above and below the crack to relieve any further pressure on the plastic from the crack and to prevent it from splitting open further.
Sand down the area of the crack. Make the surface porous and rough so the caulk will be absorbed. Load the caulk gun and apply a generous amount of caulking to the crack and the sanded surface. Fill the drilled holes. Use the putty knife and smooth out the caulk surface. Let the caulk dry.
Add another coating of caulk to the first layer, and smooth it out with the putty knife. Let it dry. Fill the tank and test the leak by watching for any dribbles or water coming from the repaired area.
