Above is a link to the Winegard signal booster that is in most RV's.
The true signal Antenna's and those like them have mixed results, mostly depending on how close you are to the TV towers and if your line of sight is decent. I have one (Moho) in my office and it does okay with me being about 15 miles from the antenna's in my city. Most other roof top varieties are going to increase the distance you can receive an adequate digital signal which is important because with digital TV's, you either get a picture or you don't. There is no snowy picture. As YODA would say, there is no TRY.
Antennaweb.org is a website you can go put your address or just zipcode in and see how far you are from the TV towers that service that area. IF you happen to camp in few campgrounds regularly, you can put in their zipcodes and get a feel for how far you are from the towers and if you want to make the investment to go with the Good, Better or Best Antenna's out there.
You can also decide if you want to go with a roof or pole mounted antenna as there are plenty of solide options that normally would be used on a house that can be mounted on a collapsible pole and attached to a ladder once you have the camper set up. Some of these will pull stations from better than the 60-65 mile range most normal RV type antennas max out around.
My new camper that I picked up this spring came with an Winegard RoadStar 3000 which is a omni directional (grabs signals without having to be pointed) and has a booster like the one I had in the first link. The manual says it's rated for 35 miles but I am picking up signals from Dayton Ohio stations that are 43 miles from my home as it sits in my driveway.
As Bob points out, the Winegard Sensor III batwing antenna with the aftermarket wingman attachment is pretty much the GOLD standard for RV's currently. It's rated at 55 miles and does better than that under some circumstances and worse under others.
Some people also will bring Jack Antenna's into the debate and I have seen a bunch of people that love them and it was an improvement over what they had and others that stated it was hype and their batwing got them more channels.
Hope all of this helps answer your questions.