You pretty much nailed what you should look into. A friend had a Class V hitch with a 3 1/2' extension and it pulled a 21 foot jet no problems. That was with a 12+' camper. Then upgraded to a Lance 10'5 camper and had the extension cut to about 20 inches by the same place that installed it in the first place. You should have no problems using that combination. That was what the Lance dealer was recommending as well.
I was originally opting for some trick double or triple receiver extension system, but found the single extension to be more than adequate with the class V hitch. BTW, make sure your extension is solid steel and not just square tubing.
Good luck.
Akamatsu