Fuses and breakers are intended to prevent fire due to overcurrent causing the wiring to heat up. They offer no protection at all against wrong voltages, high or low, in themselves. A high, or in some cases low, voltage can cause more current to flow through connected devices, and so may under the right conditions trip a breaker or blow a fuse, but in a great many situations that would not come up and potentially result in the thing(s) being powered getting damaged.
Is an EMS absolutely required? No, not in the sense that, say, a sewer hose is required; it's possible to use an RV without one. However, it's a very good idea to have an EMS and it can prevent a lot of heartache should you encounter faulty electrical hookups, low voltages, etc.