Any thoughts on how much it'd cost to restore this rig?

It is always more work and takes longer than you think it will.
Ask anyone who has rebuilt even part of an older RV.
Regardless of the money involved, here are some questions you could ask yourself:
Would you rather spend the time rebuilding the RV, or would you like to travel in one? Or spend time with family, friends?
Do you have the tools, skill, free time and access to a dry place to work on it? Or do you want to pay someone to do it?