Yes, I straigtened my frame in numerous places. There are several options.
If its just a bend on a straight section use a cresent wrench. The ibgger the better. Close the jaws so they just slide over the steel, then bend the steel into shape. You cna straighten any length by moving the crescent along the edge. You dont need to do the whole section at once.
Another option is to use a couple blocks of wood and some wood clamps. Place a board on each side of the metal, put the clamps over the boards and bend. Thsi is great for bending steel.
And yet another option is to use a piece of steel, hold it in place like an anvil, and beat the steel into submission.
And yet another option, pull out the torch, get the steel red hot, and clamp your boards on both sides.
And yet one more option, pull out the saw, whack it off, and weld in a new piece.
Now if your trying to straighten the frame, well that requires the leverage of a winch, and anchors. We can get into that if that is your problem.