I use a "cheater" box (twin 30A plugs) at home, where I have two independent 30A RV outlets, and a couple other palces. I rarely find it useful in campgrounds, for one of several reasons:
1) The 20A outlet is GFCI protected
2) Both the 20A and 30A outlet are fed by a single 30A source, so the source breaker trips if I pull more than 30A total
3) The park does not permit it (I was reprimanded at one park when they saw me using it)
Another wrinkle: if your coach has an energy mgmt system that tries to load balance to 30A, it won't let you use more than 30 when the twin tail box is used with outlets of the same polarity (phase). It is possible to modify the standard EMS to avoid this problem, but it takes some wiring knowledge and skill.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL