[Wbulk]

One question that comes to mind is why was the inline fuse holder melted. Usually, but now always, the wiring for an inline fuse is not large enough to handle an electric fan where the wiring is often a 8 or 10 gauge wire. Was the 30 amp fuse the one that was burnt.
A relay allows a low current to switch a higher current, and on a good system will have a fuse on the low side and the high current side before the relay. The 40 amp relay only means that it can handle up to 40 amps flowing through it with the correct size wiring and fuses. You can run a smaller fuse as long as the curent draw is not greater than the fuse you are using.
One thing you might look at is take an ohm meter and connect from the battery side of the burnt fuse and the other side to where that wire leads and stating wiggling that wire from one end to the other. Inline fuses if not supported can cause the wire to fray inside the insulation from vibration, which is like running a smaller wire that would blow the fuse when the fan turns on. If this is the cause you may see the resistance (ohms) get higher if you happen to wiggle the area where a wired is frayed in the insulation. On the other hand your fan could just be going.
Keep us posted as you find things out.
Good luck.
Wayne