There are so many things to consider one hardly knows where to start. Some key things to consider, not necessarily in this order include:
Design
Structural Integrity / Bodywork preparation
Insulation
Electrics
12V / 240V / 24V
Plumbing
Sink / Shower / Toilet
Toilet
Flushing / Composting / Portable
Cooking
Gas / Electric / cooktop / oven / BBQ
Fridge / Freezer
Drawer / Chest / Gas / Electric (240 or 12V)
Bed
Island / Crossways / Foldup / roll Down
Storage
Heating / Cooling
GAS / Electric / Diesel
Ventilation
Technology
Appliances
Kettle / Coffee Maker / BBQ /
Awning
Size / type
Lifestyle features
Bicycles / Canoe / Motor Bike / Library / Hobby Equipment