I've seen a lot of neat ideas for cargo trailer conversions here, trying to read up and learn what to do and what not to do.
Some Back Story:
My grandfather bought a 6x10 for camping, but he only wanted to throw normal camp furniture in it. Eventually he just threw a recliner in it to sleep in there. Eventually he decided to give it to me. I decided to make minor improvements at first: adding a step, RV-style door handle, some LED lights inside and out, throwing in the ole' college mini-fridge, and repairing the floor and installing laminate. But I figure he wont likely want to use it anymore anyway. I used it in the same manner a few times, I had a giant XXL cot with a foam roll, great by yourself but too small with company - and the thing is huge. Not wanting to wander too far off my topic, I decided to make it a camper. At first my girlfriend thought I was nuts - "It looks like a horse trailer", but eventually she found out it was a "thing" doing these conversions (thanks TnTTT

The First Steps:
I figure we already have the trailer, its purpose was for camping, lets make it a camper. I've done plenty of tent camping in my life, and I'm sure I will plenty more in the future. But the first test run after initial improvements was fantastic. I just pulled into our spot, hopped out, and was ready to relax in 15 minutes. No fussing over coolers or setting up tents. And this was all with my camping gear I had already. So I imagined it could be even better.
First Obstacle - The Bed Situation:
I took measurements of everything and played around with it in Sweet Home 3D - a free designing program. It is very basic but it was a lot easier than trial and error - moving furniture and wasting money on things that might not fit. Calling this trailer 6x10 is being generous since the interior is smaller than 60sqft. My Grand Teton XXL cot is 85" long by 40" wide - roughly 23.6sqft. Put simply this was one of the first things that had to go. I looked around a ton, most people did beds that fold up against the wall or were always there in the way. I started looking at sleeper sofas, many hours spent looking at many sofas mostly online. Running the dimensions against the trailer layout I had made. A normal sleeper sofa with arm rests you would be hell bent on getting one in Full-size that would fit, if it did fit it would barely. The only other option was to go with twin mattresses. No thank you. I kept seeing all of these sleepers having huge arm rests and seeing how the mechanism works for the bed to fold out, I thought we might save space by ditching the arm rests. After wading through futons (No thanks, been there done that - the solid spot in the middle is a deal breaker), and cheap couch-beds that come apart to rest on the floor I finally found it... Eureka! The Ikea Lycksele Lovas. It had memory foam, it was a real sofa, and it was one solid mattress. No firm spots like the futons, no springs to get poked by or get sore spots, no separate cushions to come apart when you are tossing and turning in your sleep, no quadruple digit price!!!
I ran it through the design, there was room to spare on the sides. Folded out it was still shorter than that damned cot by nearly a foot, it was wider but that space was hardly usable anyway. I could swap out the fabric if it gets ruined from camping mess. It was a winner. 5 hours later fighting through traffic to get to Ikea and test it out, we bought it and a few other goodies that I could tell would fit in well.
Plenty more details to fill in at another time.
The latest:
The attached picture shows both what is in there now, and what I am toying with doing. I haven't added windows yet, and I'm not 100% sure on the oven. I would love the sink, with an on-demand pump and LP tankless heater and shower setup outside. I like the idea but for the mean time I will probably just build cabinets with room for it to be added at a later date. I already have the mini fridge in its position, and folding camp cabinets where they are in the picture. The design is with 40" counters but I might do 38, probably not 36 because the fridge is 33 on its feet - 38-40 will allow more storage, less bending to get into it. The chairs are drawn to size, and the table is as well but it is a half-round. They all fold down.
I will add more pictures when I have time. Please let me know what you all think.
-Mark