i just did the rear floor on my 34. when i re-installed the belly pan, which overlaps the body as well, i added trempro on the joint. before i re-installed the belly trim, i made sure i have a good bead of sealer on the entire joint, sealer on all rivet holes, plus i added a small bead on top before i installed the trim.. this way, no water can get behind the trim.
BTW earlier models had indeed the body overlap the belly pan.
Resurrecting one Airstream at the time..
maybe one day i save them all