The only advantage I see with an RV or truck GPS, is that it is supposed to keep you from trying to drive under lower underpasses or routes that might cause you a problem.
Like most technology, it's only as good as the programming, and I believe that many of these are so so at best. Like you, I have seen reviews that were less than glowing. It may be that you will do almost as well with an inexpensive TomTom, if you also try and review you routes ahead of time.
The Tire Monitors are worth it, but expensive. Most folks have gone decades without them, if you maintain your tires and air pressures....but if I had the extra money laying around, I would probably invest in them too.
If the choice of your money was the Tires or the GPS, I'd go for the tires!