I agree with the dual fuel idea and use propane as my primary source when using my generator. But having an older F150 and only a 25 gallon tank I am always concerned with running out of gas when hauling our trailer. So I decided I needed to keep a gas container with me just in case.
I ended up getting a RotoPaX 3 Gallon Fuel Container which I keep in my semi-enclosed pickup bed. It is much thicker than regular plastic gas containers and it has a special mounting bracket which I installed on the rib of the bed wall.
It can be stored horizontally; but, even so, I will always keep it upright. These containers are used for safely transporting fuel on ATVs/Off-Road Vehicles and designed to withstand related abuse.
On the downside, RotopaX containers are a little pricey; but, after considering safety above everything else, I think it was a good decision not having to worry as much as using a standard thin plastic fuel container that would have a high probability of rupturing if overheated or in a wreck.
If you're looking for something that is safe, this is just another option to consider.