Many of us perhaps misguided individuals believe we have found the light...the One True Way To RV...but realize not all agree, and we are therefore evil. Evil sometimes tries to lure the innocent with gifts such as cookies or candy. The shiny red apple to Snow White, for example.
We RVers - figuratively speaking - are offering free candy (benefits of our chosen type RV) to join us on the dark side. (Good mileage in the case of class Bs, more room in 5th wheels, convenience of having your truck if you tow a trailer, etc etc)
Here's another definition from.googling: Someone might be humorously said to 'be going over to the dark side' if they opt for a preference for something not currently in fashion or not the accepted preference of their social group.