Your buddy needs to log on to rv.net and get some setup advise. I carried an Arctic Fox 811 on my F250 while towing a 20' enclosed trailer with no sway or control issues. I did have to make upgrades to wheels and suspension, but I was driving a SRW.
Is your crew cab truck a short bed or long bed? You may have trouble finding a short bed camper without an overhang. A horse trailer will have a lot of tongue weight which will take away from your payload for a camper. Are you prepared to upgrade your 3500 like I did my F250 to safely carry a camper and tow horses at the same time?