ballpark cost to rebuild engin/tranny; Roadtrek 190 popular

Generally it is only economically viable to rebuild an existing engine if there is a specific problem, such as one or two cylinders with damaged pistons. If instead you are talking about preventive engine and transmission replacement, I suspect you will be looking at the $12,000+ ballpark. It would probably be a lot easier and cheaper to find a low mileage coach with detailed maintenance records and you would likely get something with comparable reliability.