I have used the new flow-thru TPMS on two fivers for almost 2 years. They have the user replaceable batteries. I never take them off my fiver and I have never replaced the batteries yet, not sure where the information comes from that states these batteries have to be replaced every year with continuous use.
I don't add or take out pressure every often, but I also have new tires and only tow about 6,000 to 7,000 miles a year. It would not be as easy just to take non-flow-thru on and off to add air since there is a small set screw in the side of them that secures them to your existing valve stem. And remember, your trailer tires might show 85psi when the temperature is 30 degrees out, then on a 75 degree day they will measure 92-93psi. Drastic temperature changes will effect your tire pressure by 10-15%. I have never heard of it effecting tire pressure up to 40%. Maybe it would be possible if you were going from 40 below zero temps to 120 degrees, but I doubt it.
While towing the display shows each of my tires (one at a time) with the exact tire pressure and temperature. So in other words it updates and gives me that updated information for each tire every minute or so. If my temperature or pressure goes past, or under a certain limit that I have set, then the alarm goes off.