If the walls are rotted, more than likely the roof is rotted too. If you really want to spend the time and money on this project, then get the camper under roof and tear into it. You should start where you know there is decay and start stripping back from there.
If you start with the wall, and stripping them out, I almost guarantee you'll find rot on the floor and the ceiling too. Which means you'll be tearing out some of the roof and some of the floor. Don't try to patch anything until you've identified all that wrong first. Clean out all the rot, THEN being the rebuild... Floor first, walls second, ceiling third.
Personally, as suggested above, I'd dump this camper and chock it up to bad experience and get something that is ready to go. You are going to invest a LOT more time and money into this camper than you can imagine. And then, if you decide to sell it, you will not get your investment back out ... not at all. Upgrades, fixes, cleanliness, improvements, do not add value to RV. The value is based on age, not your emotional or financial investment.