These strong fiberglass-reinforced polypropylene adapters are commonly used with our fresh water tanks equipped with 1-1/2" FPT fittings.

Just screw the 1-1/2" MPT end into the tank. Our polyethylene tanks do not require thread tape with this fitting. Tighten about 1 turn past hand tight and it will not leak.

Then attach either a 1-1/4" or 1-1/2" diameter (ID) clear vinyl or green corrugated water fill hose. If using corrugated hose (which doesn't kink in smaller turns), use a little silicone caulk or other food-safe sealant as pipe dope on the barb.  A hose clamp finishes the job.