Here's a lot of information. Long post! Don't worry if none of it make sense right now - it soon will.
As an overview, typically you get all or most of the following in a Class B van:
Isolator or Battery Separator – separates house battery from engine battery to prevent a dead engine starting battery but allows all batteries to get charged when engine running.
House Battery(ies) – provides 12v power to the coach to lights, fan & water pump etc.
House Battery Disconnect Switch – disconnects the house battery to prevent discharge when you are not using the RV.
Converter – provides 12 volt DC for lights etc. and charges house battery when plugged into utility power.
Inverter – gives you 110 volt AC from your 12 volt DC batteries so you charge a phone or watch TV.
Generator – used to provide 110 volts AC to the RV. Often needed to run the coach Air Conditioner or Microwave oven.
110 volt electrical panel with breakers
12v distribution panel with fuses
Transfer Relay - automatically switches the coach from generator power to utility power.
Fridge with freezer.
Furnace for heating the coach in cold weather.
Thermostat to control the furnace.
Water Heater – propane or combo propane and electric, can be DSI (lit automatically) or pilot light (manual)
Water Heater by-pass valve – used when winterizing the plumbing system.
City water connection – lets you use campground water instead of onboard water
Fresh water tank
Fresh water tank fill
Fresh water tank drain
Waste Tank discharge valves – black tank (toilet) and grey tank (sink & shower) dumping valves
Waste hose storage
Front seats may swivel to face rear of van – extends the van living space
Propane detector – 12v propane leak detector
C0 detector – carbon monoxide detector
Monitor panel showing propane, battery charge state, water and waste levels
Propane fill valve
Propane shutoff valve
110 volt 30amp power inlet - power the coach when plugged in
Cable TV inlet
Air Conditioning (both for the coach and the cab)
Rear Hitch for towing or bike racks etc.
Exterior storage compartments
Curtains or blinds / shades
Different manufacturers do different things but that should give you a good idea of the items involved.
Browse through some owner manuals for a more detailed explanation of what does what and how to do things:
Winnebago ERA owners manual
http://www.winnebagoind.com/resources/m ... /15Era.pdf
Airstream Interstate owners manual
http://www.airstream.com/wp-content/upl ... manual.pdf
Roadtrek RS Adventurous owners manual
http://www.roadtrek.com/_uploads/manual ... _model.pdf
All this should give you a good idea of the systems involved.
Do think you'll stay in campgrounds or "boondock" either in forestry type sites or even parking lots like Walmart? Or will it be a mix of both?
Do you think you'll prefer to shower in your van or at campground showers?
They're more or less questions for you to answer for yourself. If dry camping (off-grid) you would probably want a Class B with at least two house batteries. If shower in the van you'll want a Class B with a comfortable (for you) shower.
Ask as many questions as you want.