3-4 oz glass does not have much strength in the way you would put it on a joint. It should be a 45degree angle of the fibers. Granted 45 degree glass costs a lot more. But when you use 90 degree you only have half the fibers giving you strength when on a joint like that. the one parallel to the joint really do nothing. You did nothing wrong on your trailer, the glass is just meant as a smooth water proofing layer, not to be structural. But a little movement as you found out can open a joint, and using 90 degree cloth does not help as much as 45 degree.
Now you say not having a house, thats the good thing about grinding it out is you can be water tight again quickly. Basically you go at it with the grinder until you hit solid dry wood, then slap a layer or two of glass on it. Lets say you have a 3 inch crack, you grind it out with a paddle sander on a grinder, will take less than 5 mins to chew thru a lot of wood. Then you mix some epoxy, put on a patch of glass and wet it out. your first patch should be as large as the ground out area. Then do you a slightly smaller area with the next patch. even with just 2 layers of glass and epoxy its water tight.
You can grind out a joint and have it glassed in an hour, with a few layers. Grind out the next area, and do a few layers, then add more to the first to get it close to level. Keep working on one area and going back to the previous as needed. Once you get all of them ground and filled, you do filler to get em smooth and level. Then re coat the bed liner.
edit look up biaxial 45/45 glass
edit 2 jump to about 10 mins in this video, or just watch from the beginning and pretend its wood not glass.
https://youtu.be/4XoeGV2KhGc https://youtu.be/4XoeGV2KhGc