@KSkaiser Welcome to the zone and congrats on your new ride. Sorry you got it the way you did.
As for military trucks all military vehicles leak! ALL OF THEM! One of my rules of military vehicles is when driving in the rain your right foot will get soaked! Every truck that I've owned and I've own many over 30+ years in this hobby my gas peddle foot has gotten soaked while driving in the rain.
Unfortunately because of the short (in terms of years in production and service) the M715 had there are not as much surplus parts for them as say an M37 or the M35 etc. Do a search on here as well as the web you will find you may have to get creative in order to seal things up. You may have to take a piece of the door seal to a local shop and see what you can match it up to. But for me that's part of the fun.
Good luck with your truck.