Electrical problems last night

So we are doing our first camping trip in a few months and last night were wokin up to the carbon monoxide detector beeping at us. After poking around for a minute i notice the lights are dim and the fan is slow. BTW, we are on ac power at a campground. A few minutes of poking around and a cranky wife, I decide to go outside and dx the battery, problem solved, lights get bright, co detector shuts up and I lay in bed thinking.... Anywho, I dont have a volt meter on me and dont need the battery this trip so will check it when I get home. Any thoughts on potential problems, or could it just be a bad battery? Not sure how old it is, came with camper when I bought i used last year.