I just finished my build and am no expert, but am very pleased with the way my trailer came out. I too couldn't find much how to on the subject and almost overthought the technique, but bought a bottle of titebond glue, some canvas, and a quart of "gripper" primer and used some scrap wood to experiment -- I was concerned about how hard it was to get the wrinkles out and my experiment showed that wasn't an issue ( saw posts where people were washing the canvas and ironing it) I didn't do either. I think the "gripper" primer is preferred because it is a thicker primer -- you can really put it on thick before it wants to run. I did my priming over days so it always had 16 hours or so to dry between coats. My 4x8 trailer I think I used 4 gallons of gripper (6 or 7 coats) and 1.5 gallons of paint (3 coats).
I used water based paint, but I think you can use oil based if you wish. It is exterior paint -- so it is made for the elements.
To get the seams watertight you over lap the fabric. You want your fabric seams to fall away from your wood seams so the ends of the plywood are protected. Basically I have 2 layers of canvas where the edge of the plywood is. I did my edges first, and trimmed the top canvas to the edge so my seam "bump" (about 2in wide) runs down both sides of the front and over the top. I only used one layer of canvas on my hatch and on the inside you see the edge of the canvas (I haven't finished out the galley yet). I used 1/4 in Masonite for my front radius and I can tell from the inside that there is no moisture getting through the PMF.
Some people route out the wood before so the overlap is smooth, but I didn't - you can barely see the overlap after the 9 coats of material.
Use just enough glue to hold the canvas down and to work the wrinkles out -- the smoother your canvas the less you have to sand later. You want the fabric to soak up the gripper/paint as that is the waterproofing -- not the glue.
If I remember right it took about 4 generous coats of the primer for the weave of the fabric to go away and then the finish is as smooth as you want it -- I used a roller so I have a slight texture to my finish.
I sanded any runs and after a couple of coats the few wrinkles I missed. I also would sand the seam bump back down to the canvas so the paint would build up and make it less noticeable. I think if I kept priming/painting it would completely disappear, but I got tired of painting and can live with the "shadow" of the seam.
I chose PMF because I knew I could work with it -- I too was concerned about working with aluminum (and the cost of a mistake). And I worked alone -- I didn't have the extra hands to help me. I thought fiberglass would be too messy and the idea of the fumes/respirator wasn't appealing as I knew I would be working in the 90+ heat of the late Texas springtime.
I saw a post the other day where a man is building cabinets for his van and he has a pretty good youtube video of how he used fabric screening over foam board to build them.http://www.tnttt.com/viewtopic.php?f=55&t=67373
Pretty much the same technique -- he shows it on the inside seam instead of the outside.
I don't post youtube videos, but have several pictures in my gallery and my build journal that shows the steps I took.
Hope this helps - good luck on your build - I have begun to enjoy my effort in the great outdoors!