I think you're overthinking and creating a lot of extra work and expense for yourself....whaaaaaat , whhhhyyyyy ?
On a 8 foot wide rig , a 1/2"/foot taper is only 2" @ 4' from center to each edge. Skip the side vents , they'll leak whenever you're moving or the wind is blowing.
A roof w/2x4's 16 o/c (perfect for your 14"x14" vent cutouts)and 1/2"or 3/4" plywood (osb might be asking for future problems , although they do have some with a waterproof coating now for walls and roofs) + sealed well with something that ends up white (always cooler), will be way more roof than most have and will make installing roof vents , solar , etc. way easier AND will cut down on weight , cost , time + fabricating nightmares !
The 3" of insulation will keep you plenty warm/cool and bathroom panels (FRP is awesome stuff !) on the interior , will be a vapor barrier and keep moisture form condensing up there and in the walls if you go with that there as well (a plastic sheet if using regular wood).
Don't forget thin is best weight wise....and the insulation will be a vapor barrier also.
If you still want to taper (don't forget , that will add to total length), cut the 2x4 down an inch on each end with a nice , easy to do straight cut and lay the ply on that , giving you 2 flat sections with a 1" peak (plenty for runoff, 1/2" might be enough too) , (you can even rip a little angle on the mating edges of the ply if you're worried about the gap at the peak.....I'd suggest just a little filler...bondo?if your coating won't fill it).. and then make your vent cutouts , do the waterproofing roof coat of your choice (sealing the cutouts as you go too), done on top! Walls won't need anything very thick for panels.... Insulate , inside panels and done,done!
Just my 2c worth (of spinach).