On the contrary it’s actually good for your cars to do this. Running at cruise RPM gets the engine nice and hot which will ultimately prolong the life of the engine. Driving on the highway is much less stressful than driving around the city anyway. And these are relatively recent vehicles so you should have no problem. I drive my old Honda (180,000 miles) on the highway at 70-80 mph ALL THE TIME and it keeps on trucking. Just keep the oil changed and you will be a happy camper.