Yes, I keep posting it because it's true. Z-brackets are made for shore homes, to be installed permanently on the rails. In RV you need to be able to get under the panel for cleaning, and you WILL have to do a roof rubber replacement or re-coating at some point. With Z-bracket bolted into the panel frame bottom and #10 screws through the foot of the Z into plywood, the only way to remove the panel is to rip #10 screws out of the ply.
Use L-brackets instead. 1/8" aluminum angle 2x2, 3x3 or 4x4, 6" long. Drive two or three screws #10 into the ply, with plenty of roof sealant under the bracket and over the screw heads. One machine screw goes into the side of the panel frame at each corner, you might need stainless rivnuts (aka nutserts) in the frame if you can't get under the panel - 2x2 angle leaves about 1" gap. With 3x3 angle you can insert machine screw or clevis pin from either side, no rivnuts required.
Check installations by Mr.Wizard and Bigfootford in the link above.
Mr. Wiz used a double-sided sticky tape under the bracket because his roof is fiberglass, but on the rubber you need a roof sealant.
Short vertical "legs" of Bigfootford you don't need if you don't plan on tilting, though they are handy to raise the panel higher and to insert machine screws in the frame without using rivnuts.
To hit the rafters with #10 screws - you may try. Luckily 120v panels have - usually - 65" length and most rafters are on 16" centers, so this is possible. With a flat mount and 3 screws in each bracket I don't think this is necessary though. Roof normally has 1/2 or 3/8" OSB (kind of a particle board), or occasionally a real plywood. So you don't need screws longer than 1" if you don't hit the rafters. If yo do, you will need 2"-2.5" screws.
If your manufacturer is still in business, phone them and ask to email you the drawing of roof assembly where it shows rafters. Just tell them the truth that you are installing solar panel.
Slanted to drain the sand and water off - if you install panels lengthwise, with panels on the slopes, i.e. panel doesn't cover the crown of the roof, with brackets of the same height the panels will automatically become slanted.
