Light weight is a relative term. To some, 75 lbs is light weight, to others not so light. This one gets pretty good reviews:
I don't have personal experience with this particular one, but I've found memory foam, in general, to be far superior to any "stock" rv mattress. This one is 30.3 lbs for a 6" Queen.
Don't make the mistake my wife did. She ordered a 10" from Wal-Mart and it was very comfortable once you climbed mount Everest to get into it. We've since replaced that one with another memory foam and have no complaints. Over the years we've had 6 or so memory foam mattresses and only 1 that wasn't comfortable.
The only thing resembling comfort that's likely to weigh less is a select comfort, but I'm not a fan personally.