One of the most important and often overlooked first steps when starting a small business is preparing a business plan. Many entrepreneurs who hear this might panic. And while the value of writing a business plan is often debated, those who complete them are nearly twice as likely to successfully grow their businesses or obtain capital compared to those who don’t. More on this in part three of How to Start a Small Business 101, Components of a Business Plan.
I would definitely stick with the vapor barrier under the floor to avoid moisture from breaking up the OSB. try to build the base high enough to keep it dry and allow airflow to dry it out under there. FYI I recently repaired a neighbor's shed roof that must have been partially repaired before with plywood. The OSB was fine under the shingles, but the plywood was wrecked due to moisture. I would stick with OSB. Since you live here in NOVA, details will be a little more helpful to you. I really can't use the shed in the summer without the AC on. It gets direct sun and gets up to the 90's in there with out any air movement. I am planning on eventually lining the inside of the ceiling with that thin reflective insulation to bounce back some of the heat.
_{Nicole and I decided not to go public with the cost of the build. Although we are very open about most things, we are a little uncomfortable about making an expenditure of this magnitude public knowledge. Furthermore, unless you live in the Phoenix area and plan on building the same size shop with the same patio and with all of the same finishing touches and labor requirements, the price isn’t all that helpful to you.}
Many DIY campervans have kitchen units that are directly behind the cab, sealing it off from the rest of the van. These layouts offer increased privacy and stealthiness, especially if you completely wall off the cab, and open up some space in the back of the van. This layout can be paired with a lengthwise bed, fixed bed, or convertible dinette bed. You can also place your propane and water connections right by the door, making refill easier.
_{Sadly, that’s most of my power tools and shop accessories, but it’s a growing collection. Compared to all the money I’ve wasted on small electronics and computer junk in the past, I’d say this has been, and will continue to be, a much better investment. I just wish I had come to that realization back in college, when I was probably spending $500-$1000+ a year upgrading my computer.}
The refrigeration setup in your van is an area where you really do get what you pay for, whether you’re paying in money or in time. The best options are the most expensive. The cheapest options are a pain and/or don’t work very well. And if you try to save money with a DIY refrigerator setup, you could end up spending a lot of time on installation.

Ultimate Small Shop