You can only use a hole saw if there is a center. The hole saw has a small drill bit in the center, maybe 1/4 in or so in size. That 1/4 bit is the guide that keeps the big circle saw blades in place. Without that center bit, or without a center to the hole, there is no way to use a hole saw, it will just scramble all over your work space.
You can weld a plate, then drill through the plate and the part you want to enlarge (but lets face it, a guy who can weld is not asking an RV forum how to drill a hole in metal), or
maybe attach a piece of wood there to act as a center point for the saw, wood will be easier to drill through than another layer of metal too.
The easiest is to just grind away. This sounds like a lot of work, but really should be the easiest. I personally do not have a grinder that small that would fit inside a 2 inch hole. The link given for HD's grinder will not work either - too big.
I have used a file to make this kind of hole bigger, but 1/8 inch all the way around (makes it 1/4 bigger) will be a lot of hand work on steel.
Cutting slots and bending will make a mess. This would work on thin metal, but trailer frame metal is too thick.
Consider that a jack is not a precision instrument. Maybe you dont need to make a perfectly symmetrical circle in the center-line of the trailer. Make your cut/grinding off-set, to the front, to the rear or even to the side, so just grind away half and when the new jack fits then slide it in. Hand grinding will not result in perfection. Make a circle mark with a sharpie before you begin so you have reference as when you grind you lose the original circle that was your starting guide.
