More on the UV topic...
The Home Depot/Lowes FRP panels are do not have UV-protection additives, which may be a problem depending on the environment and usage of the trailer, although in my research I've found many people report success with it.
Non-UV-protected fiberglass will "chalk" over time and eventually can even lose structural integrity under long exposure in very harsh environments. Depending on the type and formulation of the resin, it may yellow in the process or it may not. The definition of "over time" depends on how the fiberglass item is used and the environmental conditions it's exposed to.
An non-UV-fiberglass-wrapped camper that's kept outside in the weather all year will probably go some years before it gets to the point that some remedial action is needed. In South Florida, for example, due to the intensity of the sun, the time could be much shorter; in overcast upstate NY it could be a lot longer.
If it's garage-kept and only outside for some weeks of the year during camping trips, it should last a good long time.
Stored outside, it can be protected with a breathable car cover that filters light but lets moisture out; that will significantly reduce UV effects on the fiberglass.
To some degree, a quality marine wax with UV additives will delay the onset of UV damage, although wax doesn't really provide very good UV protection and needs to be reapplied regularly to have any meaningful effect.
Fiberglass is easily painted with most paints; automotive paints work particularly well. A strategy for a budget-minded build could be to leave the fiberglass bare for the first year or two (or more, depending on the environmental conditions and usage) and paint it later as finances permit. Some people may find it easier to get a high-quality finish on fiberglass than on wood, so an FRP skin would be a good option if the plan is to paint the shell.
Based on the Home Depot/Lowes pricing, which for most people wouldn't require shipping costs added on vs. the pricing plus shipping of the typical UV-protected skin products (like Filon) the non-UV stuff is about $1.00 per square foot when bought in a 4x8 sheet locally, and the UV stuff can be about 6x more ($200 or more shipped for a 4' length or 102"-wide Filon from
http://www.factoryrvsurplus.com/products.php?product_id=2122, for example), so depending on the environment, usage and budget requirements of the builder, either the Home Depot/Lowes FRP or the RV-industry Filon (or equivalent) might be the right choice.
Another alternative would be to use something such as aluminum sheet, in the same manner as I've drawn for the FRP sheet.