You first need to determine how much power you consume from the battery. Its often suggested that wet deep cycle batteries are only good for 50% of their capacity ie a 120ah battery is good for 60ah, and that is a good place to start (50% is just a arbitrary point where someone has decided its a good balance between practical usage and battery health of life expectancy. You can use more or less, it will just affect the batteries overall life expectancy)
Make up a list of
what appliances (watt, get it) you are using, and for how many hours per day you use them. Combined the consumption of those items over a 24hr period. Consumption is measured in either watts or amps, but you'll need the amps for this exercise and the amps you will mean amps/hr.
eg
You get the idea
Many appliances are measured in watts, so just convert it
watts / volts = amps (per hour) My TV is 48watts, so 48w / 12v = 4amps/hr. Hopefully the totals are less than the magical 50% of your battery capacity, or you'll need more batteries before you look at solar panels.
The size and type of the solar panel or panels will then be determined by how much total current you have calculated that you will be consuming from the batteries each day, and need the solar panel/s to replace. Solar panel output amps are calculated as the
panels wattage / panel voltage ie a 200watt panel / 20volts = 10 amps per hour. ("12v" solar panels actual voltage is around 20v ish) I would be counting on a maximum average of 4 hours per day that youll will be harvesting solar power, so that 200w panel will produce 40amps per day in the perfect world as determinced on the mountainside of a far away Scandinavian village surrounded by half naked nymphs. Its a bit like the fuel specs you thought you be getting in your new car, that the dealer showed you in the brochure. Down here in the real world, so you could easily loose 25% of that so now we have 30amps per day from our 200w panel if we have lots of clear sky and beautiful sun.
If youve worked out youll consume 30amps per day, one 200w will go ok. If you worked out youll consume 60amps, youll need 2 x 200 panels. If you want or only have room or funds for one 200w panel you have no choice but to curb your power consumption to 30amps per day. When its overcast, youll have to curb your consumption anyway because youll be harvesting less
You can do what I did and throw calculations and techno mumbo jumbo out the window and super pimp it up by dropping in 260amps of batteries, and slapping 600w of panels on the roof, and throwing another 600w of blanket panels on the ground and without a clue how much solar your sucking from the sky you only know its too much because the grass is charred around the van, the neighbours are vaporising, the tv brightness is like a welding flash and a black hole forms over your campsite