OK, let's make it "really confusing".....
First, Keystone uses "less than grade 1 plywood (cheapest bid, probably CDX, which is grade C face, grade D inner layer, grade X bottom layer)...
If we're "lucky" all of our roof trusses are 16" on center without any deviations... Think about how wide the space is for the air conditioner, and you get the idea some of them may be further apart than 16"...
Here's the APA Load Span Tables for plywood:
http://www.pacificwoodlaminates.com/...SpanTables.pdf Look at Table 3: 3/8" grade 1 sanded plywood is rated at 79 pounds per square foot when installed on trusses 16" on center....
Now, let's look at our "feet in shoes"... Most men wear shoes that are around 12" long, 4" wide, which is 48 square inches... That's 1/3 of a square foot... We have 2 feet, but when we walk, our ENTIRE body weight is applied to one foot (the other one is in the air)....
So, if you weigh 200 pounds, when "walking on the roof, wearing shoes" you are applying roughly 200 pounds to 1/3 of a square foot of "cheaper than the Grade 1 rating" plywood... So, the potential to damage the roof by walking on it is increased based on "cheaper (inferior) materials, walking with 1 foot in the air, twisting on the TPO (which causes stretching) and if you're carrying anything, you're going to increase the weight above that 200 pounds per 0.33 sqft....
Now, if your trailer is an "ultralight" model with 1/4" plywood sheet decking, you can imagine why we say, "USE RIGID FOAM TO SPREAD YOUR WEIGHT"...
Is it any wonder why nobody at Keystone will tell you, "Sure, you weigh 226 pounds, you're safe to walk on your roof, but if you weigh 227, you can't walk on it"... Then, when the claim comes in that you were "relying on what Tom told me yesterday in your CS department" and I was carrying a new air conditioner across the roof and my foot broke through the plywood... You owe me a new roof because I was doing what Tom said I could do.....
Ain't gonna happen.... You also won't find anyone at Ford that will tell you that it's OK to sit on the hood of your new SuperDuty.... Although it's probably designed to hold 300 or 400 pounds, but not in "every location on the hood"....