Tentatively I'm thinking of 2x3" 0.12" thick tube for my frame, is that strong enough for 15' long box light duty off-road with a targeted trailer weight of 3000 lbs? What if the weight was closer to 4000 lbs?
So I am in the early design stages of my squaredrop which tentatively will be 15'x6.3' without counting the a-frame/coupler using 3500lb Timbren axleless suspension, if weight comes in heavy I may go up to the 5200 lb option since I read that they recommend staying near 20% under the rating. I may be able to shorten the frame up to 13'x6.3' but as of now I'm working on a 15' box, mostly because we have two dogs one being very large and I designed a 3' wide x 6' long area in front of the bed where they can sleep without totally encroaching on the bed as I'm not the smallest person nor is my wife. We frequently camp in areas where the dogs (especially the medium sized one) will get picked off by wolves or mountain lions if they sleep outside so inside it is for them, they are so spoiled anyways. It'll be too long for anything crazy off-road wise, and I'm really trying to think outside the box to make it shorter, down to 10' would be ideal as we almost never are in campgrounds, but we're also not taking it down trails found in Moab, just rough forest service roads.