One huge consideration for what you need in your trailer is how you intend to use it.
For me the trailer is used to haul the motorcycle and my riding gear to and from the riding site and protect it from the elements - the motorcycle is clean, dry and secure while traveling. The trailer also serves as my sleeping area once I get to the riding area....and I park at a campground/resort that provides a bath house/bathroom. Most often I do not have access to 110V "shore power" - and that is fine as I do not cook food or use AC or need heat. I have not ever needed to "boondock" for extended periods of time - my rides take me to places I can get food and drink. My riding is done in a climate where daytime temperatures can be in the 90's but cool down to the 70's in the summer - fall and winter riding can have overnight temperatures drop into the 50's....that is very comfortable sleeping inside the camper without any AC or heat needed. A small amount of battery power provides the power I need to run the ceiling fan and charge my USB devices for several days.....I do have a 100 watt solar panel that I can use (and if needed I do have a small 2,400 watt inverter/generator that I could use.
If you are going to be using your trailer in hotter/colder climates or using the trailer for actual "living" while traveling - your needs are going to be far different. Having heat/AC, a water supply, a bathroom and the ability to refrigerate food and cook adds considerably to what you need in your trailer........and including your wife on the trip really makes a difference and requires a lot bigger trailer to provide sleeping areas for 2 people. Running AC, a microwave, a fridge, heat, etc. really ramps up the system you need to provide power.
If you want a trailer/camper that has AC/heat/water/bathroom, etc. that provides a comfortable living space.....and is more than just a "rigid tent" - you should consider trying to find a good used commercially made toy hauler. We bought a used one for $11,000 and used it for a couple of years, and sold it for what we paid - it wasn't a break even event as I had to install new tires, bought a cover for off season storage, and sold it with a water hose and other essentials I had purchased. Doing this will often get you a fully functional camper that is likely more reliable and comfortable than anything you can build, and it doesn't require you to spend a lot of time fabricating something. Our toyhauler was 23' long and was a Greywolf 19RR. It was towable with a 1/2 ton truck and was a comfortable living space for two people. With the motorcycle inside the only thing we lost was the dining room table - we could camp comfortably on while traveling with the motorcycle inside. It did not have a generator so we could not use the AC, TV or microwave if we were not plugged in at a campsite - however the 12V battery could power the lights propane furnace, water heater and cook top/oven for several days. While traveling to our destination we could overnight at Cracker Barrel, Walmart, Rest Areas, etc without paying for camping.
https://www.facebook.com/marketplace/item/584927901134198/?ref=search&referra... NOTE: The only big problem I had with the Greywolf 19RR toyhauler I had was the deck height was very high. Using the rear ramp was very difficult with the stock ramp as it was very steep. I had to build a ramp extension that provided a longer ramp for loading my 500-600 pound street bikes.