2008 Chevy 2500HD Wheel Question

I have a 2008 Chevy 2500HD with the stock 17" wheels looking to get new wheels. I'm wondering what is the largest wheel & tire set up I can put on my truck without a lift & wondering what it might look like. I use the truck as an everyday driver, but also haul a 30ft RV on vacation. I would like to maintain good mileage & hauling quality. Was considering 20's but from what I have seen it looks like I would have to lift the truck. Thanks for the help.