Coupler repair/replacement

I have cut off and replaced some couplers, but a 10 year old trailer? Sounds like a lot of abuse, or more likely neglect. If it was mine, I would first spray it with WD-40, and see if it loosened. (Yes, there is better stuff, but most people have a can around) Most likely it will work better, and that will tell you to save some money. Remove the guts, clean up rust and caked lube. Re-lube, put back together, and adjust.
BTW, if it is abuse, instead of neglect, might look at replacing with a eye, instead of coupler.