I've double towed my 26 ft 5er and 20 ft pontoon boat on tandem 26 ft trailer for a couple Colorado seasons. My boat w/trailer around 3,500 rigged n' ready. When I did this I was running my 96 F350 PSD CC Dually 2WD 5sp Manual. That truck has a few problems, gotta lotta blowby, prolly needs rings. It would slow some at the extreme summits some of the steeper grades round Colorado. But 45MPH gets you there eventually. As far as towing, best advice is don't take any chances, 55 or 60 tops for me when doubled up. Figure the point is to get there, not get there first. Also, make sure you can drive back out or around where you stick your nose in. I stop and either have wifey walk through or do it myself. Once I had to unhook the boat and we manhandled it back so I could back the camper out of where I shouldn't have gotten into. Wifey said, "don't do that anymore". She figures taking alittle hike and looking is easier than horsing that tandem trailer around. If I drag doubles anymore I'll get a receiver mount winch and also a front reciever installed. On the road I take my time and leave lots of room tween me and anything else. They'll keep jumping in front of you, so just back out of it some and open the gap. It isn't that they don't think, they're just thinking about something else. All my miles on the roads I've never seen non commercial doubles in the ditch, seen hundreds of singles. I always looked at it like having double the responsibility, makes you even more cautious. Suggest your first few trips be shorter than longer. Get alittle experience at it affore you head out for 3,000 miles. I usually only doubled when I had three or more days to stay, lots of trouble rigging and what not, to be running doubles on the weekend, JMHO
Cheers,
Steve J