In brief
An RV has two electrical systems. A 12-volt system runs the lights, the water pump, the heater control board and the fridge on DC; an AC system runs the air conditioning, the microwave, the hair dryer and anything you plug in. At a site you connect to the mains through a shore-power cable; off-grid you generate with solar and store it in a battery, and an inverter turns the battery’s 12 volts into 120 or 230 volts.
The single most common mistake is buying solar panels sized in watts while the battery is sized in amp-hours. Watts are an instantaneous rate; amp-hours are the storage you actually use.
Shore power: the socket tells you the limit
In North America a campsite socket marked TT-30R delivers 120 volts at 30 amps for an RV, and a 50-amp serviced pitch delivers 120/240 volts split across two live conductors, which is how a motorhome runs two air conditioners at once. In Europe the norm is a 220 to 240 volt domestic socket, commonly 16 amps, so the total load is smaller than most people expect and a European campsite outlet rarely runs a European RV's air conditioning. Check the socket before you plug in, and never use an unprovisioned adapter to draw more than the site offers.
Batteries: AGM versus lithium iron phosphate
AGM batteries are cheaper but you can only practically use about half of their stated capacity, because drawing them below 50 percent shortens their life badly; they also cannot deliver the sustained current a microwave or an inverter needs. Lithium iron phosphate batteries accept deeper discharge, charge far faster and deliver their full usable capacity, at roughly two to four times the price. For any outfit that spends real time off-grid, lithium is the system that stops being a compromise after the first season.
Solar panels, the inverter and the rest
Sizing the solar array follows a simple rule: divide your daily watt-hours of consumption by the hours of good sunshine, then multiply for the days you want to be self-sufficient, and allow about 20 percent for system losses. A 400-watt panel array feeding 400 amp-hours of lithium will cover most two-person off-grid use. An inverter should be sized for the peak load, with headroom: an 800-watt inverter runs a coffee machine but not an air conditioner. A battery monitor with a shunt is the cheapest insurance in the system, because you stop discovering the state of charge from a dead site.
Living off-grid without a generator
If you camp without a mains supply regularly, the four things that matter are: enough battery, enough solar, efficient appliances (an induction hob or a Calor gas camp stove rather than a microwave), and LED lighting throughout. A diesel or petrol generator is a fallback, not a plan, because the noise keeps you off the sites you came to.