If the top of the tank is accessible you might look into buying "inspection ports". These are usually 4" diameter fittings that act as a way of inspecting and cleaning out the interior of a tank. These are designed to be mounted in the flat top surface of a poly tank and are pretty easy to install. They are essentially over-sized bulkhead fittings with a 4' diameter plug. They work very well on top of the tank but are generally deemed unsuitable for the bottoms or sides.

Install one of them and you can gain easy access to make repairs from both sides. Those ports are available at chandeliers and stores catering to boating and sailing enthusiasts. Some RV places catering to people doing repair work may also carry them. Poly tanks are commonly used on boats, both power and sail, and RVs. They are commonly used for diesel fuel, drinking and waste-water.