Do diesel fumes get into the TT when towing?

Not a valid concern...unless you are standing still, idling for extended periods with all the windows open in your trailer....

no

No smell at all except when it re-gens, then you get a "sulfur" type odor. Otherwise, not a problem. However my roommate from college once worked at a gas station in the mid 80's and said a car/truck came in that was a diesel that had a bumper sticker on it that read "Diesel Fumes Make You Horny". That being said, is perhaps why OP's wife may have a concern.

I don't even have one of those new fangled smokeless diesels but no, It doesn't. You should keep the windows shut while driving or a lot of dust gets pulled in from my experience.

Take her down to whatever dealer you are thinking about. Test drive a new diesel and have her give it the sniff test. If she can smell anything it's imaginary.

Just look at the inside of the tail pipe. It will be clean and shiny.

There will be no diesel smells in the camper. And, with today's blends of diesel fuel, it doesn't smell like it use to 40 years ago. You can stand right behind a new(er) diesel pick-up truck and never smell anything.

If you, or your wife, sees someone driving a pick-up truck (diesel) and it's belching black smoke, making a stink, it's because the owner has added motor oil to the gasoline to make the truck pump black smoke out the exhaust pipe(s) for show-off reasons. And that stuff ... yes ... does stink.

But if the (newer) trucks are used according to the way they were manufactured, you'll never smell anything, unless you stick your nose right in the exhaust pipe.

True, however, if your wife hates the smell of diesel, she will smell it regardless. She will never be happy with a diesel.

Most factory exhaust system go out the side and away from the trailer. If you mess with the exhaust you are on your own. I can't imagine why the fumes would be any more of a problem in the trailer than they are in a car traveling beside the trailer and I don't hear any complaints about that unless the truck is no longer to factory specs.
What my wife fusses about is if I get any diesel fuel on me while refueling and it is hard to get that stuff off.

I tow my TT with a 2005 Ford diesel and have NO diesel fumes in the TT.

Bill