I do agree that there are a lot of interesting ways that the top of an AS can be accessed. I think how one choses to do it would depend on the nature of the task and how heavy they are.
I installed 3 new Fantastic fans on top of my 1973 31' AS. I also weighed in at about 240lbs at that time (have lost some weight since then). I found that as long as I was carefull where I put my weight I could work up there just fine with some soft rubber knee pads. Just make sure you put your knees on or near the body ribs and you will be fine. You can easily find the ribs by looking at the rivet pattern. Also I just pushed with my hand to see how stiff things were before I placed my knee in a given spot.
Also make sure you wear shoes that wont scratch the aluminum. I have a pair of Lands End slip ons that have been awsome for working on the AS. They are completetly waterproof and extremely durable. They also sell for around $30 which is a bargin in my book considering what I have put mine through (and they are still going strong). Check out the following at Lands End. I bought mine a while back and these new ones seem a little different - maybe better. I think you might be able to buy them at Sears stores now too.
I have a folding metal ladder that I used to climb up. I used some pieces of foam pipe insulation to wrap around the top part of my ladder legs (one piece for each side). That way I could move the ladder any where I wanted without having to move my padding first.