Great idea Scott, and hope you have a few takers.
I am just going to throw up some negatives though, but only because it's easier to pick on the negatives than put the extra thought into being constructive. Hopefully as the day progresses I might be able to give it more thought and give you some positives

Most of your possible clients would be "owner builders", as such they will probably have lots of ability and experience and be able to nut out most of the design in their head or have access/knowledge of how to draft it through their workplace or similar.
Other potential clients will be at the other end of the spectrum, no idea/learn as they go type DIYers (and good on them for having a go). These type I expect will have limited ideas of what they really want and will be figuring out the design, features, specs as they go.
I am probably somewhere in between - basic trailer building skills (jack of all trades, master of none

), access to a drafting program, and nutting out the design as I go. As such whilst the ability to draft the plan to fine detailed specs is good, I find I have to keep redesigning on the computer because a fitting (for example) won't work quite as I thought it would so have to modify some design details to accommodate something different.
You could say that ironically because the CAD system allows the design to so fine tuned one small change (ie slightly different water tank size or shape) may require a substantial rejig of entire sections.
Also a big benefit of DIY custom built items is the free labour which typically means that you get a custom made trailer for about the same money as a production line model (although I am sure most of us hope to make more savings it doesn't quite eventuate

). Adding your drafting costs in would possibly eleivate the cost a reasonable amount. And not suggesting your charges will be super expensive, but there is no point you doing it unless you can make it worthwhile for yourself. No one expects you to be a camper trailer charity service.
I see possible customers for you as being a niche inside a niche. DIY builders who have had a variety of trailers over the years, that know exactly what they want and how it is going to be built (and not waiver from those ideals), but want a detailed plan to refer to as they put it together.
Alternatively you could offer a "design" and drafting service. But this would rely on you having an in depth knowledge of all the different "off the shelf" parts (suspensions, tents etc) plus an in depth knowledge of all the custom built, possibly owner built parts (suspensions, tents etc.....) and being able to include it in a design that a person with a very low skill level will be trying to put together.
Peter