Solar system does not consists of only solar panels and wires, like some people might think. In an off grid system (where you want to be completely independent from the grid) you need to store solar energy somewhere. The only feasible solution so far is to store it in batteries. To charge your batteries correctly you need one more device which is called a battery charge controller.
Main job of a solar battery charge controller is to prevent battery from overcharging. Good battery charge controller also prolongs battery life by completely charging it and gradually cutting off the electricity flow from panels.
Look for models which are Pulse Width Modulated (PWM), not shunt controllers or “Low Frequency Pulse Width Modulation”, those utilize older technology which transforms into loss of voltage and shorterned battery life.
Solar charge controllers which have “maximum power point tracking” feature also allow slightly faster battery charging. The thing is that solar panel actual power output depends on temperature and amperage (how many ampers do you draw from it).
Maximum power point tracking feature allows solar panel charge controller to draw maximum power from a panel in your current conditions. When you will be choosing one, don’t accidentally pick one of the charge controllers for wind generators, they are a bit different.
So, to summarize, the off-grid system is a bit more complex than on-grid, because of a necessity to store the gathered energy somewhere. Solar panels harvest energy, solar batteries store this energy and solar charge controller makes sure your batteries are charged fast, efficient and prevents them from overcharging.
As with any other equipment you have a wide variety of choices for solar battery charge controller. Depending on your budget and requirements you need to pick the appropriate one, so it will fit your system well and you won’t spend money on unnecessary features.
As an alternative you can setup an on-grid solar system, but it works only for domestic applications and not very suitable for RV trailer for obvious reasons.