Any rv that has a 50 amp hookup has 240 vac available on the inside of the rv. They may not use it but they have it. Many don't use it because when they connect to a 30 amp pedestal, the 240 vac dryer or stove top wouldn't work. So they use a 120 vac dryer or stove top. I agree if you hook up to a 3 wire dryer connection of old you have a big problem. The good news is all dryer hookups for quite some time have been 4 wire.
06 Millennium Prevost
15 HD Limited Low and 16 HD Triglide (DW)
13 Avalanche Black Diamond with Cruiser Caddie