I use solidworks, fusion and sketchup quite a bit. While its true you can do a lot in sketchup, particularly in pro, sketchup is not a parametric modeler, I.E. the underlying architecture of how it organizes and relates items in sketches makes it really non ideal for things like designing a camper. What you'll find is that as your design comes together, you'll need to shave an inch off of something here, move an inch there, change an angle. Infinitely easier in solidworks to do those kinds of manipulations than in sketchup.
That said, if all you are trying to do is a quick rendering, and maybe a guide to use when you are building, I.E. not generate drawings for a CNC etc, sketchup is probably fine. They also have Mogs in their 3D warehouse so you have something to start off of (I used to have a Mog, saw the light. You can call me an anti-mog evangelist
I'm not nearly as good at fusion, so I can't comment on how easy it is to use, but its also quite a bit cheaper. And even with my limited experience, I'd say much better for camper design than sketchup. Solidworks ends up being ~3k. I bought it for mine because the design hours I'd have into someone else doing it would have exceeded that cost, but its hard to justify it for a one off.
There are other programs out there, Rhino etc to consider as well.
The other thing to keep in mind that others have mentioned, the learning curve...I had only sketchup experience when I started solidworks. It took me a solid two months of tutorials to get decent at it. Just another thing to factor into your considerations. My first month or two with SW were filled with a lot of critical mistakes in how I organized my model that resulted in doing alot of things over again.
