For most owners of Class B camper vans we recommend the Hughes 30 Amp Bluetooth Surge Protector with Auto Shutoff. Another good choice is the Progressive EMS-PT30X. If you have a lithium-based van, a more generic surge protector such as the >>>> may be a better choice.
Most class B vans have 2 electrical systems. One is commonly 12 volt and runs off of a bank of batteries. The other is commonly 120 volts AC (the same as house current) and runs off of a glorified extension cord connected to an outlet that supplies power to your van. This is called shore power. At a campground, shore power is typically provided by an electrical pedestal that has different shaped outlets for 30 amp and 50 amp service as well as a switch (circuit breaker) to turn the power on. A Class B van typically uses 30 amp service. WIthout going into the weeds, an amp is, very roughly, how much energy you are using. For example, a typical RV air conditioner uses around 10 amps and a microwave another 10. When you are running both at the same time you are using 20 amps. Since you are using a 30 amp service, the total of everything you are concurrently using in your van must be under 30. Bigger RVs have larger energy needs and typically have a 50 amp service.