Solar prewire only means that a pair of 10 guage wires are run from a point at the roof (exact location depends on model, but the owners manual tells you where) to a location near the central monitoring panel. Then another pair of 10 gauge wires are run from the monitor panel location to the 12V
terminal block near the front of the trailer (this is where the battery(s) connect as well as all the 12v
lamps/accessories and the charger/convertor.
To make all this work, you have to purchase solar panel(s), build or buy mounting hardware, drill holes in roof and mount the solar panels, drill another hole in the roof, fish out the solar wiring hopefully under the hole, connect the wiring to the panel(s), purchase a solar controller, install the solar controller near the central monitoring panel (ie has readouts and usually one or two switches), connect the wires from the solar panels to the solar controller, connect the wires from the 12 V terminal block to the solar controller and finally connect the wires from the solar controller to the 12V
terminal block. In addition, it is recommended that you install a 12V surge protector to (similar to a whole house unit, but rated for 12V systems.
All of this is usually beyond what owners typically want to undertake as a DIY, but it can be done. I am a EE and pretty mechanically inclined so I did mine myself. I have 2, 55W panels and a MPPT solar controller. I designed the mounts myself and did the fabrication. I probably invested 20 hours of so, start to finish. Understand that solar usually means that you have simply made your batteries have a longer usage between "real" recharging. On a really clear, sunny, summer day, I can get about 45-60 amp hours of charge (assuming that there are NO daytime 12V loads. As part of my solar system, I also added a Tri-Metric battery telemetry system that allows me to actually track how many amp hours I have withdrawn and replaced. This gives us the information to make the most of the solar capability.
Once this is done, the solar panel will charge as many batteries as you have installed, given enough sunshine and time.