when you write Decline i am assuming you mean Up Hill? Incline is up hill and Decline is Down hill? All good.

It actually sounds to me like you may have a Clutch Issue for it to free spin on re-acceleration uphill towing a load??

Can you feel your clutch spinning?

When you are chugging up the hill towing the load and cant get to 60kmh what gear are you in? what are the revs like?