Starting an overhaul on '77 Holiday Rambler Wide World

Ok you guys have me lost....lol. The only reason I was considering an on demand type of water heater (like those smaller Eccotemp type) is because mine is leaking...from the bottom. I'm assuming from rust.

So...first things first...can they be fixed? I'd like to do this the most economical way whether it's pressure tank or something that replaces the one I have now or ??