As stated before, you need to address what are your needs.
Fortunately there are LED light which produce good (bright) light with little power draw.
Just to list a few items that you might power:
lights (interior, exterior)
fan
usb ports
refrigerator/freezer (12VDC, dual power frigs- 120VAC/12VDC)
monitors (CO2, CO, smoke, fire)
computers -- laptops, tablets, DVD players
routers & signal boosters
heater ( propane/natural gas/diesel heater with ignition system and fan)
water pump
radio - AM/FM, satellite, ham, weather
TV
There are other things like
CPAP machine
heater -- 120VAC
microwaves
blenders
coffee machines/grinders
electric blankets
Granted I am getting pretty over the top with possible usages.
The general idea is you need to compute your Amp-hours. Generally speaking a lead acid battery, one can use half of its listed capacity. Lithium you can use 80% or so.
You than need to compute how many Amp-hours you will be needing per day.
For each 12V appliance, you can multiple their draw in Amp by the number of hour per day you use it. LED lights can have very low draws -- 0.2 to 0.6 amps per hour. So run for 2 hours is 0.4 - 1.2 Ah. From above list, one of the bigger draws is a ceiling fan or refrigerator. Bigger fans like MaxxAir or Fantastic Fan have relatively large draws and are often run for long periods. The big MaxxAir 7000 is rated at 5 Amp. So it can only run 16 hour on high and your battery is dead.
Inverters can be computed the similar way but you have to multiply by 10 in the USA eg 120V @ 1 Amp = 12 V @ 10 Amps. Of course that requires 100% conversion. Most invertors have lower efficiencies. So you are consuming even more amps. That is why you want to use direct connections for phones or computers, to eliminate the conversion to 120VAC then back to 12VDC and the loses that has. Most computers do have connectors that plug directly into a 12VDC socket though it is an extra accessory. There are DC TV.
All of this means you need to plan what you want to do. If all it is a single LED light run for a couple hours, you are great. If you have all of these, you need to find a way to recharge while you are traveling/camping be it solar, shore power, generator or from the towing vehicle or some combination.