I recently took possession of a newly built 38' gooseneck concession trailer. The first issue that jumped out to me based on my limited experience is the size of your water tanks, especially when you are selling fountain sodas. Also, you have to consider the source and quality of water when doing that. You will be required to get water from an approved source.
After getting your water from the approved source, you have to get it to the fountain at the correct rate of flow (additional pumps w/pressure tanks), and it will likely need to be filtered of chlorine after leaving the tank. I learned about the chlorine filter the hard way when at my first event, the soda tasted like it was made with pool water.
Also, I do not see any notes about an ice machine. We go through an incredible amount of ice, and the cold plate on your fountain will consume far more than you think. Even with a 200 pound ice maker, we are constantly using bagged ice to keep up with demand.
We carry about 200 gallons of water in our tanks, and have 200 gallons more in a support vehicle tanks. (support vehicles could be their own topic) The support vehicle tanks were purchased out of necessity in the middle of a long event where we ran out of water.
Even with the fountain, you will also need to carry plenty of bottled water and cans of soda/gatorade. You are building something with a big footprint (as I did) and it is going to be critical that you maximize profits in every way possible, as your costs are going to be much higher than others. You might be paying 3x what the guy is paying who erected a 10x10 tent paid just to get into an event. Odds are, you can't afford to just be sitting there selling BBQ (the lowest profit item on a BBQ menu).
I don't want to speak on the BBQ business, as I am not necessarily a BBQ guy. I do enjoy it, will sell it, but usually prefer to leave that to guys who do it exclusively. One thing I have noticed about many food trucks and concession trailers at events is the bottleneck at the window. When you talk of cutting the brisket in front of the customers and also selling them fountain sodas out of that same window, it immediately concerns me.
One last bit of advice, you really need to work hard on the business plan and be realistic before you build anything. I have had to modify my menu numerous times to get into events because many are looking for product diversity. I knew that going in and built my business to be agile in that sense. If you are going to commit to a BBQ business, you need to figure out how you are going to compete with the thousands of other BBQ businesses that exist. Also, be prepared to be turned away because they already have three other BBQ guys. I can't tell you how many people think the food business is as simple as making great food. That is just one component, and often the easiest. Getting in position for someone to buy your food is the challenge.