With respect to your dealer vs. DIY install for solar, that is totally dependent on your and your boyfriend's skill level and abilities. The dealer will cost more since you will be paying a labor rate of $80 to $100 an hour plus the dealer needs to make a profit or he won't be a dealer for very much longer.
A more important question that you should be asking yourself at this point in the process is: what are you trying to accomplish with your proposed solar set up?
If you are simply trying to keep your battery topped up and you will be hooked up to shore power most of the time, keeping your current single battery and going with a limited capacity solar panel will be fine (cost can be well under $1,000 fir this and DIY is relatively simple.)
On the other hand, if you are planning on being "off the grid" for extended periods of time, the stock Airstream battery setup will not work, especially if you plan to run your furnace for any length of time. In this situation, you will want to add a lot more battery capacity and a corresponding amount of solar to keep them charged up. The cost of batteries, solar panels, controllers and installation for this scenario can range between $2,000 to $10,000 depending on capacity and whether you DIY or work with a professional.
I would suggest that you follow the following basic steps:
1. Determine what you want to accomplish.
2. If you decide you want to do extensive "off the grid" camping, then do an inventory of your energy needs (figure out how many Amp Hours per day you need to power what you want to run.)
3. Identify and/or design a system with the capability that will help you achieve your objective and/or find a professional installer that can help you with the design and installation.
You will need a lot of information in order to navigate this path. Use the "Search Function" on this forum to find threads that are relevant to your questions, look at web site of bloggers who have done this (here is a link to a good blog with lots of information in this subject: http://www.technomadia.com/2014/12/solar-planning-conducting-an-rv-electrical-consumption-audit/)
buy a book or two on the subject, and keep reading!
Keep in mind however, that you will not be able to run your AC off your batteries and solar setup. If that is important to you, then I would pass on the solar for now and look for a generator solution.