When you start looking at hardware details, all-in-ones often use premium laptop components (for management of power and heat loads), and are comparable in performance to entry to mid-range deskside boxes. Thus all-in-one vs laptop depends on how important is a larger screen, versus the questions of where will I store it, how long does it take to set up, and do I really have space for a 20-26 inch display screen. For CAD the screen space can be important, but there also can be a question of how much processing power you need.
You are also working against compromises of capability for what you want to spend..
My apps are photo/video, and when my Windows 7 deskside died trying to do defective OS updates, I decided to switch to Mac. Balancing cost vs capability, the smallest iMac (22.5" screen, four-core I5 processor, 8G ram and 1 terabyte drive) beat out all the price-competitve laptop options, which would have given me 1/3 to 1/2 the processing performance and a 13" screen.
You are likely thinking Windows. When I was shopping, Dell was selling 22-inch all-in-one closely matching the specs of a base iMac for about $ 200-300 more, price competitive with their upper range laptops with matching processor performance and 15-17 inch screens. So it is still a balance of screen size vs portability.
For RV use, I'd be tempted to use the laptop instead, and still using a 17-inch Dell with dual-core i5. I want my work backed by "uninterruptible" power, and for a desktop or all in one in a RV that means a backup UPS or a laptop, as a laptop carries its own backup power.