As others have said, if the engine is gunked, injectors dirty, then there are assorted additives that MIGHT help.. on the other hand some of them may make it worse as well.
Sta-Bil and Seafoam are good for when the rig is not going anyplace soon (like winter storage) and I do use it if I do not plan to buy gas for oh say six months.
Used it last winter for one tank full (Filled up around October, added Sea Foam (2 cans) did not buy gas again till something like March or April (forget which) Did drive the motor home a number of 20 mile trips in that time, Up and down hills (SC Foothill conntry) So the engine got a good workout.
And again when I blew the engine (One can, half a tank) here in GA.. Since I'm now using only about a gallon a month (Generator exercise) till I got the 10 grand saved up (I am at 4 Grand as of this afternoon).