We decided on the Sidekick 8 by Dynamic Bicycles. It's got the larger 20" wheels, half the weight as the similar sized folding bikes at Camping world, compact enough to fit into the allotted space on the truck, and they have NO chain. That's right, no chain. Instead, they use a Sussex shaft drive system paired with a Shimano Nexus 8-speed internal hub (range of 21 gears). It's all geared specifically for use with 20" tires, so the pedaling and effort required is the same as on a larger bike. The only thing you have to get used to is the steering differences those 20" wheels make.
We setup a compartment with bracing to hold the bikes securely in place and prevent them from rubbing on anything as the truck rumbled down the highway. So far it's worked well. The bikes didn't fold up as small as they said they would, so I have to remove the seat post each time we store them - not a big issue though. Here's a pic of them stored (we've since tweaked it more and added our helmets, etc., but you get the idea):
We've already had them out several times during time off at parks and campgrounds, but this recent heat has kind of put a damper on things. It's still nice to know we have the means to get places (that perhaps would be to far to walk) when we need to...and besides, they're fun!!