My 1974 Shasta looked about the same as yours, Just wash it good, knock off all the loose stuff, make sure it is clean and free of mold and such. Then after it has dried, do all the seams, front, rear, around all the vents and a/c, and the sides. Go to your local big box store, and go to the roofing section, and you will see the elastomeric roof coating! It comes in gallons, buy 1, and read the directions, it is easy, and does a very good job! Once it has dried and cured, you will be set for many dry years! Enjoy!