Fresh water holding tanks in RV's are usually not fiberglass. Plastic welding usually works, but the leak may be a fitting connected to the tank.
Some people drop part or all of the belly cover. Some cut a flap leaving a hing on the front side of the flap. Seal it after work is complete using high strength all weather tape.
However, the location of a leak and the source of the leak are often not near one another, so at least dropping part of the cover will allow you to find the source.
Dutchman Kodiak Cub KD176RD 2018
Nissan Pathfinder 2015