Thank you both for your feedback...the reason that I am looking into the extension is because I would like to take 3 bicycles with me and the camper as well. Currently I have a rear-mounted bike rack for the 3 bikes which just latches on to the hatchback of the Outback, completely independently of the hitch. With the trailer connected to the hitch, there is not enough room for the 3 bikes. However, with the 18 inch extension, there is enough room for the rack and the 3 bikes.
My initial solution was to look into roof mounted bike racks, and I was ready to purchase 2 Yakima individual roof bike racks (expensive at about $170 each) and perhaps carry the 3rd bike either in the camper or the Outback. The funny thing is that the guys at the bike rack store talked me out of this option (even though they would have had a revenue of about $400 from me...). They are the ones who told me the better solution was the hitch extension which I could get at an auto parts store down the street for less than $100. One of them, an experienced trailer, said that "everybody gets extensions". (I should add that I am only 5' 6" and it is not all that convenient for me to use roof bike racks.)
Are the potential issues that Fairtax4me described lessened at all by the fact that my camper is relatively light at under 1600 pounds?