where do i fill with water coolant on an isuzu truck?

Get your handbook out, hopefully it will walk you to it

You fill at the coolant reservoir.

Usually located near the firewall on the passenger side of the engine compartment.

Most often this is a white plastic tank with levels marked on the side for "full hot" and "full cold".

try behind the driver seat on the floor, if you look under the cab you will see it

radiator or overflow tank