Make sure you incorporate the business, and get insurance. You are making a product that could have a significant likelihood of lawyers getting involved if something goes wrong. Also make sure you really are covering all your costs.
There are some industry standards for electrical and water plumbing, I'd recommend following them. There are some standards with teeth for the plumbing of toilets, but if you don't have them... The nice thing is most manufacturers of toilet systems have recommended installation setups and they will meet the standards. For the most part RV construction rules are few and far between.
Look up what it takes for a certificate of origin in your state, and serial number ID plates. Just have a minimum sized batch made by a maker. That will likely be 100 or so. It would also be good to stamp the serial number permanently into the frame in a couple places. A set of metal stamps could be used for this.
For liability reasons, I don't think I'd make a TD without brakes. The reason is many people want to pull them with small cars. The state maximum weights for trailers without brakes were set when cars weighed allot more than the light weight cars we have now. If the user uses it without properly hooking up the brakes and gets into an accident, your lawyer can use that fact to extract you from liability or at minimum reduce your liability allot.
Don't install trailer hitches yourself. Send the buyer to a hitch place to do it. This goes along with never telling or even hinting to a buyer they can tow it with a vehicle not rated for towing.
On the trailer lights wiring, FOLLOW THE COLOR STANDARDS! That will make it easier on anybody repairing or modifying the TD later. Places like Waytek Wire sell wire cables that already have the right colors needed.
http://www.waytekwire.com/item/WT701/TR ... CONDUCTOR/ You can even find places that will make your trailer wire loom for you. You just tell them what you want where and how long to make each wire group. PS, always make the wire going to the TV a separate segment. All sorts of things happen to that wire, and if it gets damaged, repairs will be much easier. On the wiring, a wire color code sticker in the junction box where the trailer harness connects to the hookup pig tail would be a good idea. Custom vinyl stickers can be made for relatively low cost.
Thinking on stickers. Warning stickers are likely a good thing to have. If you see some RV maker have a specific warning sticker, then it likely is a good idea to have a similar sticker. Yeah, I'm one to think warning stickers are a waste, but then I have what allot of people consider to be a very high level of common sense. Many people don't, and some use lawyers to make up for their neglect of knowing the pitfalls. BTW, I have a series of sculptures that are called the Decidedly Deadly Series. They will drop you dead if you touch them when they are powered up.

Yeah, no warning stickers except the one that says "STOP! NEVER PLUG IN!" on the 50 Amp 240V power cord...
Think about your designs from building and accident damage repair perspectives. As you are waiting for the paint to dry, you can do things like build the wiring harness for the next one. A wiring harness is something you can do in the kitchen while the garage is stinky from paint fumes.
Lastly, this list is only a starting point.