I'm not really understanding your question - you said you understand MHs doing it but not a 5th? What's the difference except that in a MH you don't have to leave the MH to get to the bed? In a 5th, you do have to go outside for a few feet.
Are you talking about the difference in leaving the MH vs the 5th in the parking lot while you might leave for dinner? If that's it, there's no difference there either that I see?
As for sleeping, if you're talking about noise, WalMarts and Travel Plazas aren't a lot different than a lot of RV parks that are just off of the free ways or built on cheaper land out by the rail road tracks - the type of RV park you would spend the night in as you're passing thru someplace. Truck Plaza's can be a little noisy at times, but I've always found the WalMarts to be quiet. But noise level would be the same for either a MH or 5th.
Bill