Doable or overly ambitious?

I don't think the question is whether the trip is doable, but whether it would be enjoyable. That seems like far too much driving and too little time camping, IMHO. I'd go somewhere closer and have less of the limited vacation time spent driving.

Buying & operating an older RV of any kind isn't necessarily hassle-free, either. Saving money on a 1-time trip may not be sufficient reason for such a purchase.