Keep in mind motors and heaters/lighting are going to be different. Motors will pull more current starting as most of these are universal, but that current will clear the cb as it's only present for a few cycles. But as someone noted before, when the motor bogs down, it pulls more current which could cause the breaker to trip. If a power tool falls in the current rating for the truck, 3.3-3.6 amps, you should be able to use that for light use, or even a motor rated a little higher should still work as the actual current used is based on the load.
Heaters are different as it is purely resistive load. Therefore no inrush (starting current) and will always run at it's rated load. I'm not sure what size the circuit breaker is, but it would be your main limiting factor as there will be some tolerance in the inverter. A 400W inverter should be able to produce a little more than it's rated power. Your cb should keep you from blowing any fuses in the inverter as that's the last line of defense against frying it, the cb is also going to protect the wire as it's also a limiting factor based on it's current carrying capacity.
Long story short, electrical engineers get excited talking about this stuff and probably more info than you'll ever need. Just remember motors will pull more current under increased torque, and heaters will always pull their rated load. I would guess there's probably a specialized cb size at around 4A, so keep your heater load under that and you should be good. You can most likely plug in any power tool and pull the trigger and it's going to spin. That's because the motor is under essentially a no load condition so it will not pull much current. As you apply load, the motor will pull more current and if you approach the current rating of the circuit breaker, you're gonna trip.
The smaller the coffee maker the better as the heating element will be smaller.