The normal motorhome windows use tempered glass. If you do decide to use residential windows, you'll want to order them with tempered glass. You might find a used door on craigslist. Sometimes people part out a rotted RV and you can find windows, doors, and appliances. There might be an RV wrecking yard in your area as well. The RV specific windows are designed to seal well to the skin of the RV and they have a lower profile frame that doesn't protrude like it might with a residential window. Even the cheapest RV manufactures use RV specific windows so there might be other advantages as well.