A 25×30 RV garage will give you approximately 720 sqft of interior space, which is big enough to be able to comfortably house many class C RVs such as the Forest River Sunseeker.
Prefabricated kits have the benefit of being quick to construct, thanks to the fact that the separate parts that make up your building are manufactured elsewhere, then brought to your location for assembly.
In some cases, you could have your RV garage up and running in under a few days.
Steel is an excellent choice in materials because it is durable, robust and weather resistant.