I bought a HF 4 x 8 trailer, flipped the axle with stronger ubolts. to lower it. My tow car is lowered already
I was able to extend and strengthen my tongue with an old boat tongue, It's 2 x 3 box with a welded receiver on the end flipped the stock receiver mount over and notched, drilled and bolted them together, was also able to bolt the back to 2 crossbars of the trailer. I feel comfortable with it and it gives me about 18" longer tongue
Plans are to build a 2x4 frame which will notch around tops of tires and give me 9 foot length and 55" width, WIll have a cutout over the tires with the original fender covering the exposed part .then flooring on that. WIll make a section or two of the floor lift up for some storage below, will go down 6-8" or so. After looking at all the tear drops and foamies, I decided to go simple box shape with a angle on the front. I am not needing extra insulation so just going for plywood walls for now,
All I really want is a place to crash at night and some storage. I have a couple of Solar vent fans I can put in the roof. I have about 50-60 watts of solar panels and several AGM batteries. I will have LED lights inside and out for lighting and decoration. I will be able to attach an awning off the side for shade and seats. I will put a 5000 btu AC unit in either back or front depending on how weight distribution falls out Walls will be approx 1/2 ply, with stiffeners glued on outside if needed. flat triangles 1/2" thick glued into corners and 3/4 " for front and rear walls to stiffen it up. I plan on glueing all joints and 2x4 crossbars at a few key locations.
As it comes together I will know what it needs. I work better in real time than from plans
Not worried about extreme temps, but the foam ideas are intriguing as well as canvassing. I want to make this simple enough to actually finish in a few months, but durable and possibly upgradeable later. Canvassing the outside sounds promising, but haven't learned enough yet to consider it seriously,
My goals are comfortable bed (54" x 72") storage for most of our gear, electric lights with a switch and a phone /USB charger, solar fans and a battery fan for night with a small bank of batteries , couple of windows that open with screening. Place to attach awnings to outside and then a couple of poles. I have a 5 gallon jerry can for water and 1 for fuel, may mount them on outside to fix weight distribution..........
Crude paint drawing to show the basics. Bottom right is my crude attempt to devise a bell crank to mount a shock horizontally under the bed and dampen the bounciness I can do basic welding and different holes in the bell crank will change the ratios on the shock

We currently go camping for up to a week at a time, with just a tent and what we bring in the station wagon. We will have an enclosed awning to set up our Kitchen in, Not planning on cooking in this camper