Very odd question about lowboy trailer I'm sure.

To put things in perspective, I have a 7' x 14' enclosed trailer (enclosed part is 14' total trailer length is about 17' including the A-frame and hitch). This is a generic, nothing special about it utility trailer. Curb weight is 2,200 lbs. So triple that to give you a frame length of about 42' would give you a trailer curb weight of over 6k lbs. Realistically when fully built/furnished/loaded up you'll probably be around 20k lbs or more. Seeing how you plan on pulling with a pickup, I would say you'd be looking for RV type axles with electric brakes? Personally I would go a little overkill on the trailer axles (better braking power).

As for frame design, everyone seems to think a boxed frame is the greatest thing since sliced bread. Personally I prefer C-channel construction as it can't rust or corrode from the inside where you can't see it.

 

And pulling it with a truck like this

 

Bigger horse or less house, your trying to build something requiring something along the lines of a class 6/7 truck and planning on pulling it with a pickup truck. Your understanding of a trailer is way off, a lowboy/rgn type "Semi" trailer is designed to have a good portion of the load carried by the tractor. If you build on that design of a trailer the tongue weight will be 1000's of pounds over what your towing vehicle is designed to "carry" let alone drag. You will need a heavier frame, axle, suspension and brakes on the tow rig to carry the weight and a lot more cubic inches under the hood to drag it. Yes I know all about the kinds of horsepower and torque numbers that can be made with a 24v Cummins Turbo Diesel or a 6.0/7.3 Powerstroke yadda yadda yadda. Those numbers look great pulling on a dyno but in the real world making those numbers to drag a house across the country will drag your wallet into the other house, the poor one.

The reason you see all these tiny homes being built on bumper pull (car hauler) type trailers is because of size restrictions to still be classified as an RV and no I don't have a full understanding of those. And the second and bigger reason is to keep the floor height as low as possible to keep as much headroom as possible for all the lofts and crap they cram into them. A 2 or 3 axle gooseneck (5th wheel) style trailer with spring over straight axles eats up so much of that precious headroom that they cram those tiny "lofts" into. Yes there are gooseneck/5th wheel trailers that have a very low deck just like a car trailer but they are designed to haul a car not the weight of a house and everything in it. The amount and weight of the materials needed to make it strong enough would make it too heavy to be practical. There is engineering that goes into trailers, they have to flex a certain amount to keep from breaking, be just rigid enough to be strong and strong enough to not break all while keeping the weight down so someone can actually put weight onto/into it.

On to the next issue, as the length of the trailer increases the weight increase in not 1:1 length:weight, the longer the trailer the heavier the materials must be to maintain strength so if your looking at a 18' car trailer that weighs 2000# and figuring you can get one 36' at 4000# your likely off by close to a 1/3 because the actual weight will be closer to 6k due to the heavier materials, axles and components required for the strength and longevity of the trailer.

You talk about slide outs, a raising roof system, stabilizers etc. All that adds A LOT of weight and complexity. Mark my words, all those sliding joints in something that flexes and moves going down the road will #1 give you all kinds of operational problems and #2 will leak. My family used to be in the RV business and an insider joke in the industry is the only RV in the world that doesn't leak is the one that hasn't been built yet. Your essentially looking at trying to build a house and have it be an RV. My advice would be to buy an RV, save yourself the headache of trying to reinvent the wheel and enjoy it. When your done trying to do the tiny house revolution lifestyle sell it or keep it to use as a camper and build a house that stays in one spot.

Yes all these tiny house shows popping up all over TV glamorize this fad/trend but the engineering going into the design of these, at least the ones of any quality, is left out of the story. Why do you think the prices being thrown around are so high for a 240 sq ft house, it's not the $10k in materials from Home Depot, it's the engineering and custom built trailers making them so expensive. If you try to use an "off the shelf" car/utility trailer it will flex so bad that your gorgeous new tiny home will sell destruct leaving the driveway returning itself to a pile of materials. If you build a trailer without the proper engineering you'll wind up with it so heavy you can't pull it or so stiff it'll break into two or more pieces the first time you hit a section of freeway with separated concrete joints not to mention the hassle of trying to get it licensed to pull out of your driveway.

Find yourself an older class 8 tractor and an older furniture van then build your house inside it. All the design/engineering etc has been done for you and as long as there's a flushing toilet, permanent cooking appliance and a few other requirements met you can tag it as an RV and not blow up your wallet trying to pull a house with a pickup truck. Or save yourself a ton of money, pick up a larger 5th wheel RV on the secondary market, pull it with your pickup which is what they are both designed for and join the tiny house revolution that's been going on since the first campground was built in Bocca Raton Florida 70 years ago. RV manufacturers have invested billions of dollars squeezing every inch out of a box to maximize living space and storage, why try to reinvent that, embrace it. I lived in a 28' 5th wheel that could be pulled by a 1/2 ton truck, didn't have to climb a ladder to get to my bedroom, could walk around my bed, stored all my precious belongings and was comfortable. I watch these shows where these people are spending 10's of 1,000's of dollars to wind up in a space far less user friendly and far more cramped than my 28' 5th wheel was that I spent a whopping $6k to own.