Showing where the water finally comes out is not too helpful. Can you find the source of the leak? If the water was leaking into the shower pan from the shower head supply, then it should be going down the drain. If the water is going under the shower pan and then coming out by the drain then the leak could be elsewhere, and also doing slow damage to your sub-floor.
There is nothing special about the camper being fiberglass and any decent RV shop should be able to handle this. If they would only look at it they might be fine with working on it. If the commode is the problem, its a standard marine model so any RV shop or Marine Service Center can do it.
Frankly, I would consider this to be a DIY repair except that it is under warranty, so why should you.
But there is a caveat. I am quite sure that your Scamp has Uponor ProPex piping
. It is a PEX piping system that uses an expander tool to enlarge the plastic rings that go on the fittings. If the repair requires that tool, be advised that they are pretty rare. I called a few plumbers around here looking for one who uses the Uponor ProPex system and no one had the tool.
If you are lucky, the leak will be in a part that does not require repair to a ProPex fitting (such as the commode flush mechanism), or the repair can be made using standard PEX crimp rings which any plumber can handle, and most anyone else with a rented tool and a little homework.
But you have to find the leak first.
Note: This reply was edited due do a misunderstanding on my part.