Paul, if you really want to carry a hard top carrier on the roof, I'd call Phoenix. I've seen other posts where Phoenix has sent owners the diagram for the roof structure so they can mount solar panels, etc. Also, there are posts from Keelhauler showing how he had Phoenix add another cross bar to the roof rack configuration so that he can carry his kayaks up there. Kermit may have suggestions for stowing gear on top. You may be able to add standard short rails and cross bars the same as you would mount on a car. Whatever you decide just remember down the road that your rig is now taller than what it originally was and may not fit under all the bridges and overhangs you could go through previously.
I am up & down on my roof all the time. It is very strong. Here is a picture of the rack I made from ridged 3/4" conduit. I attached floor flanges to the roof and made sure at least 2 of the screws went into the 1" Aluminum frame used by Phoenix. Roof sealant works well and has never leaked. In the past 20 years I have done this to 3 different RV's.
My canoe on top.
Kayak on top