check the gas lines, you can usually smell if there is a leak, run everything from the stove to the AC flush the toilet, run the slide outs in and out, anything that moves needs to be tested. Look for signs of rodents in the light fixtures, seems them little bastards like to dig in the ceiling and usually will leave evidence in the lights. Run water through the entire system to find any leaks, if your water pump runs for even one second with no faucets open that means there is a leak somewhere.
Check the tires, the roof, the windows, anything that might be worn out.
Maybe the best bet is to buy from a dealer, usually comes with some guarantees, and they would have gone through it pretty good.
Then have fun, my family spends close to every weekend, and more in the camper during the season, my kids love it more than I do. It is a great family activity that I really feel sorry for the kids that don't get to experience.