They are the same basic product, though obviously different brands with various unimportant (for the general operation) differences in appearance and construction. Basically they just split out the two legs of the 50A plug to the two separate sockets, and are generally safe (ideally, in my opinion, there should be a 30A circuit breaker for each of the sockets).
I would not suggest using one of these and then putting on a 30A to 15A adapter and plugging an extension cord into that to operate your various kitchen things. In terms of voltage and so forth it's a fine setup; the concern is that the overcurrent protection would be 50A for your 15A circuit, and it would be pretty easy to overload the 15A extension cord dangerously, particularly with a few cooking appliances operating at once with multi-tap outlets or cords.
Your alternate proposal is a bit better in that regard (though I'd still be careful with the extension cord plugged into the 30 to 15A adapter--a 12 or preferably 10 gauge cord would be best) and should work out okay. Some pedestals are wired such that the 50A breaker acts as a main for the entire pedestal, and the 30A and 20A are slaved off of it, but with a 30A RV you should have no problems with overloading it even if the other outlets are also in use.
Another viable option might be to use the outside outlet(s) on the RVs for some of the cooking equipment.