When Bill Swails talked about the design of the Earthroamer ER-XV, he said he was building a larger vehicle for couples or a family with children. The size of the ER2K wasn't large enough for two people to use for an extended trip, and still be friends.
I think the original ER2K is still for sale. I see it in photos of the Earthroamer owners' gatherings.
The original ER2K was a box on the back of a flatbed pickup. You can get Alaskan Camper to build a pop-up camper to any length you specify with or without the cab-over.
The RC Willet Co will built any of their Northstar campers, including the pop-up models, for use on a flatbed. They have sold flatbed designs in Australia for several years. Here's one example
http://www.freedomoffroad.com.au/850sidedoor.php
If you don't need/want a hard side camper:
Campa USA built a Tacoma with tent camping unit on the back.
They called it the Expedition Vehicle System. There was talk
about building a similar setup on a full-size Dodge Power Wagon.
Adventure Trailers has a new trailer with clamshell design in the
works. It sounds like a good candidate for mounting on a flatbed, too.
Chip Haven