Because 48 volts travels through your wire with less line loss. The MPPT controller gets every drop of sunshine into battery charge. Go to Morning star and look at FAQs, there is a good illustration there. MPPT is a good thing, but it is even better the larger your system. You will find that every advantage counts in your solar system, this is just one of those advantages. With the controller I have, I can expand if need be, it is programmable for custom settings with dip switches, but if you want you can download a program to adjust the morningstar tristar internally for your batteries.
There are a couple of MPPT, mine is a little pricy but morning star has some for less. This is an area where you can save a few bucks.
By increasing your voltage and decreasing line loss, I was able to go #10. But this is all predicated on how far you are going with your line. I went from the front of my rig to the washer plumbing vent then down into the basement ( we don't have a washer). My run was a little longer than what a 10 ga should run but within tolerance. There is nothing missing as far as power loss is concerned. What I got upstates is downstairs too. So, I like it. I know that there are people on youtube that will talk about their larger wire. But it all comes down to the math. Look at the data provided for your current, and the number of feet you want to travel with it, and you will find the right wire. Dont second guess it. I have been told 100 time my 10 ga is not big enough, but my volt meter and results say it is.
BTW, my taxman says that you can get a tax credit for installing a solar system on your home and that a travel trailer is classified as a home by the IRS. So you might want to check into that. according to him you spend 2K you get $600 credit. Sounds like a good deal to me.
You should buy a charger that will accommodate your input voltages, but hook the panel in series. The voltagedoes not matter as long as you get the volts to the solar charger. Ohms law dictates that volts X amps = watts. Your solar charger will process that into 13 to 14.5 volts and X amps to charge your batts. Your job is to get the voltage and amps there. High voltage, less amps, smaller wire, better solar charger= great charging for great batts.
I also found them on Amazon. Why do you run 2 in series? If you would run all in Parallel you would get more amps. I was thinking of about 400 Watts on the roof but in this case it would be 432 watts. Why a MPPT controler and not a PWM? I was thinking about a Morningstar TS-45 TriStar 45 Amp Charge Controller 12-48V PWM. What wire size did you use from the roof to the controller? I was thinking #4.