Do you have a need for increased capacity or does your tail end sag to the bump stops? Most common reasons for fishtailing are improper weight distribution in the trailer, overloaded trailer, too short of a wheelbase on the tow vehicle, too much weight on the tongue, and/or speed, or your truck is simply too small to pull it safely.
I am not sure how they would help with MPG.
On edit, Your truck's tow capacity is 5k-8k lbs depending on how configured. Remembering to add occupants and gear to subtract from that number. I am not sure which heartland Mallard you have but a 2018 mallard 32 for example has a dry weight of almost 7k and a max weight of 8600 with a hitch weight of 560.
Again, not knowing all the details, but there is a good chance you are at or over the capacity of your truck.