A trailer mounted tank is a great idea if you already have one. This is especially true if it's also used for hauling other things. It would have to be large enough to deal with the loss of space due to the footprint of the tank. If you have to load and unload the tank, it poses the same problem as loading the tank into a pickup.
My neighbor uses a similar approach. He mounted a 30 gal poly tank on a push cart and rolls it up onto the utility trailer he uses to haul around his mowers.
Buying a road ready trailer and dedicated tank, specific to this purpose, would be a far less economic solution, especially when you consider ongoing registration & insurance fees. The money might be better spent on a skid tank large enough to accept local fuel delivery.
The best fuel storage and filling solution varies depending on usage, storage space, budget, DIY capability and how much junk you can repurpose. One size does not fit all.