A bit more information is required. What model and year camper do you have? What light(s) are you having issues with? Does anything else not work?
My first suspicion you have a dead battery. There are are a number of parasite power drains on the battery. If it is just sitting and not connected to shore power it will discharge within a few weeks. This is not good for the batteries health, and will shorten its life. Do you have a multimeter? If not I strongly recommend getting one. If no, ask a friend or neighbor and have them check the battery voltage.
I suspect you are new to campers, and you have a TT, but this is true for any camper, with slight twists. You have two (2) power systems within your TT. A 12V DC system and a 120V AC system. Your lights are connected to the 12DC system (just like a car). There is a converter within the power distribution panel that will take shore power (120V AC) and convert it to 12VDC. It will charge your battery and run your lights when connected to shore power.
The battery is usually at the tongue, or under the step on a class C. Many campers also have battery disconnects, but not all. If you have one, the factory switch will be red handled. The factory switches are usually located inside the cargo hold closest to the battery. Aftermarket could be anywhere, but often placed on the battery box or tongue.
There are a few fuses you should also locate. There is a fuse near the battery. Sometimes it is right at the battery, sometimes in is located where the 7pin wire harness transitions to the standard 12 volt wire as it enters the TT. Look near the tongue. FYI you might have two fuses, one for the TT power feed and one for a power tongue jack (they are known to blow fuses at the end of travel).
In the power distribution box there are two main fuses. One is for "Reverse Polarity"
if the battery was connected, and even for a quick moment, with the wires crossed you will blow this fuse.
As Kirkelli stated, if you are at a CG, it is common to flip the circuit breaker off at the shore power post prior to connecting and disconnecting the power cord. But your lights should still work off of your battery.
Tip: If you are not using the TT for an extended period and it is not connected to shore power you want to disconnect the battery. There are a number of items like the propane/gas detector that will drain the battery within a few weeks. Also note on some campers there are a few things that are not disconnected when you flip the disconnect switch, and these items will also discharge the battery completely.
Hope this helps and happy camping