stingray1300: you are spot on, hence my comments regarding roof rack and vehicle roof loading; it's really hard to get the static and dynamic load ratings from the truck canopy suppliers; I tried and only a few gave numbers. Another critical point missed out earlier is the effect of a big, heavy RTT on the handling of a vehicle. I have seen broken bar towers first hand (fortunately not mine) due to an excessively over-loaded roof rack. I went with an ARE CX HD cap with an internal aluminum rack to which the cross bars attach; no load on the canopy top at all, good for 550lb dynamic, not that I'd ever put that much up top.
As you said" learn the facts"; I said "do your homework"; same thing. Most cross bars are rated for around 70 to 80lb each; my Rhino Rack HD2500 cross bars, about as "HD" as you can get, are rated for 165lb for two, 77lb for one (dynamic). these bars are for extra storage and sea kayak transport up front on my metal cab roof. Oddly, Rhino don't recommend using the 2500 systems for RTTs. I've been a Thule guy for over 20 years, sea kayaks and bikes, never a RTT so this is new territory for me. I have no doubt that the manufacturers have a pretty big safety margin built into their specifications. What Thule system do you have?
It's interesting that the cross bar manufacturers all have similar web tools to specify an appropriate cross bar system by car/truck make/model/year but none of these tools ask what the intended purpose is. Maybe as more of us buy RTTs this will change.