When you're buying an RV- whether it be used or new- you are taking a big risk personally and financially.
There are so many aspects to take into account when purchasing an RV. Like buying a car or home, having it inspected by a certified professional takes a lot of uncertainty out of whether or not you are buying a quality product. A mechanic will look at a vehicle to examine it's roadworthiness. A home inspector will look at the livability of a home.
With an RV, you're looking at a home on wheels: you need someone who is able to go point-by-point over the aspects of both. An RV inspector will give you an unbiased, professional examination and review, which can highlight issues the RV you're looking at may have now, or in the future.