i have partially restored/updated both a vintage 'slide-in' truck camper and a small vintage travel trailer and there is one basic rule for what you want to start with....always:
Little to no rot or water damage......serious golden rule on this.
This is the 'rust' equivalent to classic vehicles in terms of the work effort and money that you will be into in order to remedy the problem.
By all appearances, that trailer shown is past its prime...and I agree with the other members in that you can get a better example to start with.
You will pay more money...but you will not spend the next 2 years of vacation time learning how to rebuild a roof, walls, rotted floor etc.
Here is our latest trailer....not big but easy to deal with.
All good
Coley