It's what the realestate industry keep saying - location, location, location. Over the 38+ years out here I've had six brand new vehicles. The longest resident - 1982 Chevy 3/4 ton pickup - 13 years.
WITHOUT exception - all six vehicles looked like brand new - top & underside - when traded in. I have never specifically washed the underside of any of the vehicles. Actually - I seldom wash my vehicles. After a thorough washing - the vehicle is completely covered in dust & volcanic ash within 24 hours. I wash my vehicles to remove bird poop & cow doo.
Anyhow - the other side of this picture. I gave my 2002 Toyota pickup to my son in 2012. The running boards had dropped off - from rust - and the complete underside showed signs of heavy rust within five years.
I live out in the country. The County Highway Dept keeps their winter sand in heated storage facilities. The never use a speck of salt with their sand and never use liquid highway deicer. My son lives in Spokane. I swear, on a stack of comic books, they mix their winter sand - 50/50 with rock salt. They also use some type of "miracle" winter deicing fluid on the intersections.
I make it a point to drive my new Taco Wagon into Spokane, in the winter, only in the most dire of circumstances.
My new Taco Wagon( 2018 Ram 2500 Power Wagon) is completely undercoated - side to side, front to back. Time will tell if this is a good idea. All I know about this undercoat - it's a LineX product. I take it in to LineX, in Spokane, for maintenance check, annually. Up on the lift - lightly pressure wash the entire underside - then they spray a light coat on the compete underside. Just one of the benefits of an extended vehicle warrantee.
No winter deicing products ever used plus extremely dry conditions = very little rust on anything.