Welcome. It will add roughly 75 pounds. If your existing battery isn't brand new you will want to go with two new batteries, otherwise the old battery will kill the new battery fairly quickly. It is best if you go with two 6 volt batteries (wired in series), but if you must go with two 12 volt batteries in parallel make sure you connect the (+) to the camper to one battery, and the (-) to the camper to the other battery. Always charge them and use them together, NOT one then the other. You actually get more total amps out of the batteries that way due to something called the Peukert Effect.
With any camper you must make sure to use your cutoff switch to disconnect the batteries after a trip otherwise "phantom loads" in the camper will kill the batteries in about 2 weeks (maybe 4 weeks with 2 batteries). Once per month reconnect the batteries and plug the camper in overnight to top them off, then disconnect the batteries again. Also top them off overnight before a trip while also pre-cooling your fridge.