Vortec should be a gen VI block which is good for 200K miles. Gen VI block has a larger pan, better oil flow and cooling, and roller lifters. Those are the goodies required for longevity.
The other consideration is that it needs to be driven regularly to keep the seals lubed, and prevent the inside of the engine from rusting. I knew of a '77 Revcon with almost 300K on it. The guy had only replaced normal wear parts. Unfortunately the coach developed a propane leak and blew up, or who knows how long it would have run.