I'm not clear what "a market for commercial overland vehicles" means. Are you referring to factory prepared camping vehicles with off-road capability for sale? for rent? Or companies that organize overland travel experiences? Please tell us more.
Certainly there is a market for the equipment to make a vehicle more suitable for adventure travel. Just look at the ads in the Overland Journal, or the list of site sponsors on ExPo. And there is a market here for specialty vehicles that have a high level of off-pavement capability, such as the Jeep Wrangler Unlimited, Dodge Power Wagon and Ford Raptor. No doubt these vehicles will be driven on the road for 95% of the miles they travel, but they are remarkably well-prepared right off the showroom floor for off-pavement travel.
A small number of custom adventure travel vehicle manufacturers exists in USA today. Unicat Americas, Global Expedition Vehicles, Sportsmobile and XP Camper come to mind, and Earthroamer is back from bankruptcy.
[Edit: At this writing, Earthroamer LLC is still involved in bankruptcy proceedings. A new company has been created by several principal members of the original Earthroamer. They are building new camping vehicles using the original Earthroamer molds.]
If/when the USA economy picks up again, I expect other companies to join the party.
There are several USA and Canadian companies who construct campers for the adventure travel market. Just attach to your truck and go. Four Wheel Camper, Alaskan Camper, Adventure Trailers, and Campa USA come easily to mind in this category. There are many more.
Suppliers of off-road components in USA are starting to advertise that they offer full preparation of adventure travel vehicles. For example, Adventure Trailers now offers truck conversion and preparation, in addition to sales of their trailers and Flip-Pac campers. Not all of us want to build an off-road vehicle, just use it.
Off-road racing teams are served by a community of fabricators, including some that build support vehicles. These can also be used for adventure travel. The work of Rob McElroy, who prepares F350 chase vehicles for the Dakar Rally, comes to mind. See
http://www.rallypanam.com/trucks.html
I don't think that any USA rental company will allow you to take their vehicles to Mexico and points south. With some prior notification, I think it's possible to take vehicles rented in USA into Canada. So the possibilities of renting a Jeep and driving to Tierra del Fuego are limited today. Within USA, you can rent vehicles ready for off-pavement adventures from Farabee's in Death Valley, Moab, and some locations in Colorado.
None of this could exist if there was no "market for commercial overland vehicles" in USA.