Okay, space is a concern, but how many panels can fit on an RV roof exactly? Let's say, the width of a standard RV is around 6 ft and the length is 20 ft. We end up with around 120 square feet. But there is a TV antenna, AC, forehead area, an outdoor unit... In the end you have about 50-60 square feet for panels, which is enough for about 6 modules. Of course all these numbers vary depending on your particular RV, but it's important to consider them before purchasing panels.
When space is limited, it makes sense to go for the most efficient panels
to generate the maximum amount of energy from a square foot. Monocrystalline panels are the best in that regard. Polycrystalline modules are a middle-of-the-road choice: they offer relatively high efficiency, but cost less than monocrystalline panels. Thin film panels, to the contrary, aren't well suited for the job. While being very light, they have the lowest efficiency and the shortest lifespan – 10-15 years.
The most popular panels for RVs are 100W. However, 300-330W panels, which are widely used in home installations, are also great for trailers and it's better to opt for them if you need a powerful array.