For travel on roads, promarily, the Marathons in the D-load rating are a very good thing.
But i know what you mean about bad roading and off roading from first hand experience. If you will have to do this to an extreme, and a lot, then perhaps a flotation size tire will work better. Flotation tires as those made by BFGoodrich will definitely work better when low air pressure is required to get through sand or desert terrain. You will definitely get more sway on road, due to the taller and much softer sidewalls,and you should take a look at the wheelwells for clearance.
If, however, you won't need extreme flotation, then a Marathon will be fine.
The profile and sidewall construction actulally makes them very good sand tires, just not very tall.
The tires seen on the popups you mentioned are marketing stints, in my opinion. There is very little reason for an extra course mud tire on a trailer.
BTW, i am not a tire expert, just happen to have been through baja's sand quite a bit with my 5600lb tradewind.