If it is a "plate in a box" mount, the plate may have it's corners rounded off by now or the corners have pushed the box apart or both.
Try the old trick of torch heating the bolt cherry red and letting it cool... this is really effective in breaking loose frozen bolts ..
... penetration oil and undercoating burns - so have a water hose or extingusisher handy ... then....
If the plate has caused the box to bulge, a "C" clamp can be used to close and hold the box together to prevent the plate from spinning.
Try to jam a steel rod, i.e. screwdriver, thin chizel, etc. into the box to try and jam the plate so it won't spin.
You could try to drill a 1/4" hole through the bottom of the box and plate and then put the butt of the bit in these holes to "key" the plate to keep it from rotating.
It may take a combination of the above.
Any of the above methods may cause the box to tear out but if you can't get it out by now it is time for the last resort anyway ...
Last resort is to rebuild/replace the box and/or plate.
Easiest replacement would be to torch the bolt, pull old plate out and replace it with another one (tapped square of steel)and close the box back up - weld if necessary.
A nut inside the FRAME is a whole different story.
I'm sure we will see solutions for this situation posted..
Parts Mike once mentioned a fancy new type of insert that works to fix in-frame skid plate nuts... can't remeber the name of them ...