Melissa,

Americian La France is a great unit. First check for an identification plate for the manufacturer of the tank. You can review further by checking with the original owner of the unit. One of the largest manufactures of poly tanks is Poly Tank Inc.  They manufacture for American La France and most other apparatus builders including Pierce, E-One, and the list goes on.  Most leaks show up in seams and can be difficult to locate, A leak at the seam in front of the tank may cause water to run the length of the unit and show up  at the rear.  Direct fill inlets fitted to the tank without a flex pipe, could crack a weld. It is not usually  helpful to apply a sealent to the outer serface because of the flex of the tank during driving. The tank is designed to be removed from the top at the hose bed, its a lot of prep to remove but most of the time to fix it right ya need to remove and inspect and locate the problem. Make sure you contact the manufacturer for correct repairs. Poly Tank Inc has always been responsive to their customers. A small crack in a weld is progressive and will only get worse.