The TNTT forums are great. So are the suggestions by jscherb (who should quit teasing us all and just put Trailtop components into production).
When I started thinking about building a trailer a few years ago, I started by going through the two links at the top of this forum's page: Favorite Pics and Collection of Trailer Builds. From first skimming through the threads, and then reading the threads that looked interesting, I started saving ideas using Evernote. And I started collecting links from inside the threads themselves for the sources of parts. This of course took months, but the winters are long, cold, and dark where I live, and doing this basic research kept me out of bars and away from people with loose morals. It is partly a process of refining and distilling the concept down to what you want.
The "How-To" is found in the threads themselves, as well as the sources for parts and materials. For instance, the Trailtop thread is invaluable for info on fiberglass, the art, the science, and the tools. As you read through the threads, you will find tremendous info on the materials you will be using. For instance, if you are using wood, you will find detailed info on getting it waterproof and watertight. If you are going to skin with aluminum or FRP, find builds that use those materials and find the tip, tricks, and techniques. Insulated structural panels are used not only in the trailers forum, but also in the Other Expedition Camper Forum. Electrical information on both wiring and powering your electrical systems are found in all of the Expedition Camper Forums and in the power Systems:12v, Solar, Gen forum which is a sub-forum of the Expedition Support Equipment and Adventure Activities forum.
Personally, I think three of the best builds that approximate what you are considering are
the MUTTT build,
the Crow's Wing build, and the R.O.A.M. build by Titanpat57. You have have to get the link from his signature in any of the posts he has made. All three of those builds are epic, as is stomperxj's original Sawtooth XL build.
There are no real concrete answers. After I read and read and read some more, I decided to buy the plans for the Sawtooth XL. That worked out for me as I made some changes here and there, but I save myself the time of planning out each little detail. Jess Neal (stomperxj) had done that for me. Plus, when I was stumped, I could email him. It worked for me, but since you already have a platform (5x8 utility trailer), you will be looking at something considerably larger.
Best of luck, and please report back what you decide, and then start a build thread!