I have no doubt that there are probably sites that have ONLY 50 Amp.. I just haven't seen them. Seems to me that would restrict usage to POTENTIALLY only 50 AMP equipped RV's, unless you had the dog bone. So, they couldn't rent to a 30 amp guy, unless he thought ahead, as OP is, and came prepared.
I will certainly make a mental note the first time I find a 50 amp only site. I've only been to state parks and campgrounds in GA, FL, AL, LA, MS and Texas (and certainly not all of them by any stretch), but I know that where I have been, if they have 50, they've had both 50 and 30. In fact, I was at one that said if your RV is equipped with 50 AMP, you must pay the extra $2 for that site EVEN if you plan on only using 30 AMP. I didn't quibble for $2, but Im sure someone would.
Mike