I am loathe to disagree with Don, but I tried valve stem indicators that looked
just like those he suggested, and one failed on me. It ruptured at the top and let all the air our of one of my inside duals. Worse yet, on casual inspection it looked "green".
Now, it may have well been a different brand, so maybe his are perfectly fine.
I am not one to pay a lot of money for "peace of mind" or "cheap insurance", but I wouldn't travel without a TPMS. When we towed a big fiver, I had a blowout on the trailer that I didn't know had occurred until we happened to pull over for a break or something. Similarly, we could easily lose an inside dual on the motorhome or on the toad and not know it until it causes major damage, a fire, or loss of control.
I believe that most blowouts are caused by leaking tires overheating. I really
don't want to a blowout on one of the front tires of our Bulgemobile.
So for the last six years we have had the Pressure Pro system on our motorhomes and toads. Aside from safety, there is the added convenience of being able to check the pressure on all tires from the driver's seat. There are now several good alternatives to the Pressure Pro.
It's your money, though.