That sounds like the same ol' problem most Winnie owners have. The epoxy gives out and a bracket or even an entire fiberglass body piece can end up on the ground.
What I did when I had both front and rear body pieces come loose from the epoxy (though nothing fell on the ground) was to drill holes for 6-32 Stainless flathead screws through the brackets from the backside using a small Li-Io battery drill so in most of the places that had brackets it was pretty easy to find space for the drill. A 90 degree drill would get them all I'm sure.
After the holes were drilled, used a simple rotary counter sink so the screw head is flush with the surface. Inserted the screws, added a flat washer and bolted them down using a plastic insert lock nut. At that point, I could just dab the screw heads with paint to hide them but they are so small, really no reason to do that IMO. In 3 years of driving I've not had to revisit the repair at all. Holding fast.
As far as your missing bracket goes, you could fashion a new one based on the one you still have, or have a machine shop do it, or contact this salvage place, Visone RV
and see if they have one.
I know many people go to the extreme and try to use epoxy again but I didn't...here's my blog article about it when I had the front hood get loose: Body Work