Hello and a huge welcome to the Seamly2D Forum, @WSimmons.
A while ago, we had someone wanting to make a pond liner on the forum, which may help you a little to see the diversity of the program - if for nothing else, then at least to refine your pattern into geometrically and scalably correct proportions. (I hope this is the term I’m looking for.)
One of the things that I have on my bucket list is to buy an old, beat-up caravan and redo its interior - or build a Gypsy trailer 
I’ve used Seamly to make cake boxes, cushions, quilting designs and sewing patterns, so I do know that it can do anything easily and elegantly without having a multitude of tools that one needs to learn to get the job done. And since you have something that cuts your patterns onto fabric/chipboard will make it even more useful to you.
In your case, you can choose to create your own (Unknown) measurements in SeamlyME or you can create them in the Variables Table to keep them “inside the pattern” for easy changing for different projects using the same shaping (by changing the measurements, the pattern will adjust accordingly).
There are many questions that have been replied to, here on the forum, and lots of info on the Wiki. However, we are always around if have any questions.