I use circuit breakers for essential things like the brakes. I use fuses nonessential things because a breaker can open and close indefinitely either hiding a problem or creating other problems in the case of a shorted circuit.
You may want a larger fuse than 40 amp. Typically a 400 watt inverter has a peak of 800 watts which translates to 66 amps. When I installed the 400/800 watt inverter in my RV I believe the instructions called for a 75 amp fuse. In this case it is permissible for the fuse to exceed the wire rating because the inverter will limit current to a continuous 400 watt but needs to be able to peak at higher current without burning the fuse.
