I did a mainspring on a driver side awning. Not the big awning. Basically what is needed is to drill out tht rivets holding the spring assembly and to remove the old assembly and replace it with a new assembly. Then wind it up and attach the arm to the trailer. There is a fair amount of tension.
As late as last year we ordered parts direct from Zip-dee. Last month when we called we we were told that Zip dee does not sell parts directly and to,order through a dealer.
And are you sure you have a main spring problem, as in a broken spring? Another thing that causes poor retraction is the roller,on the none spring end of the awning. Had to take my assembly out of the roller tube and drive the axle out with a hammer and clean and lube to get it to work. Al corrosion is larger then thr raw Al it replaces so the bearing just seizes up.