Stiffening the rear suspension (with bags or any other means) can/will help tow "better." Fuel savings? lol, not sure where that notion would come from.
And cost being "expensive"? Under $400 for a quality set of 5klb bags is not much or any more than other options.
Want a cheap/free option that you can try?
Before hooking up, clamp some wedges between the lower overload leaf and the rest of the spring pack. Aka, poor man's stable-loads. That will stiffen the suspension some, by engaging the overload leafs immediately.
If you only need a little bump in support, you can make some semi permanent ones for about $30 total. I like using plastic felling wedges and dado a little groove for hose clamps and clamp them on. Can make free ones out of wood or hockey pucks first to try it out.