100W solar can be $150, yes, but you also need a 8A controller and some cables, this is another $100 or so, depending on how long cable. Small panels are usually 17V so it's 6A, not 8A. In cheap controllers extra volts over 14.4 are not converted into extra amps, you are losing them. A good sunny day is equivalent to 5 hours of full sun, and if you tilt it and point South, after all losses you could get 20-25A a day. In theory, this is enough to live off the solar indefinitely if your energy audit is correct and you only draw 20AH a day. On cloudy days or with some shade - hard to tell - could be 10 AH a day, or 5 AH. Lean it to the tongue on the South side or strap with bungey to a short step-ladder. The cable gauge from panel to controller doesn't matter, at this low current. The output of controller you connect with booster cables to either both batteries (preferable way) or just the battery that you are using at the moment (because it needs charging the most). Setting up and taking it down will take some time, this is a drawback of portable system.
With 2 batteries and minimal energy needs you will last 5 days even without solar. With one battery - only if your panel is in direct sun AND it is sunny.
