WELCOME to LMF...
You are asking about the PTO drive belt... But if it's slipping going UP a hill, I suspect it is the drive belt for the hydro pumps...
If that's the case, you need to remove the PTO belt from the electric clutch and then the drive belt on the small pulley above that.... this belt drives Both hydro pumps, just check closely the path before you remove the belt... take a few pictures to help on replacing it...
But before you remove the belts... check that the Idler pulleys and tension springs are in good shape...
a pulley or a spring failure will allow the belt to run loose, causing slippage... :smile:KennyV