Those who are kicking and screaming about how wrong this is are over the top.
Conversely, the argument that you aren't saving THAT MUCH money running 30A 4-strand wire verus 50A, 4-strand wire (it's still 4 stands of fairly heavy wire)...
It's that small savings versus "are you sure that some day, you might really want to have the full two-leg 50A service".
That argument has some validity... It will definitely cost a lot to upgrade then, so why not go for it now.
I'm sure there's some confusion in this thread that 30A 120V single-leg RV's would require 3-strand wire and probably would have some bigger savings. (Along with the classic 30A 240V drier plug where if you shove hard enough, you can plug in a 30A 120V camper and HOPEFULLY just trip some breakers.)