There are 2 types. Inside the tire and valve stem. The inside the tire systems provide temp as well as pressure, so are 'more useful', but are a pain to install and maintain since you have to remove the tire from the wheel to install it. Also, they tend to be more expensive.
The valve stem models are easy to install (and for someone else to uninstall). Some block the valve so you have to remove them to add air. And one wonders how susceptable they are to damage. If I recall correctly, Doral is one of the better examples of this type.
I heard of a company once which offered you the choice of inside tire or stem mounted sensors, and their stem mounted sensors were 'on the side' so you could add air without removing the sensor. Unfortunately, I have not been able to find this company again.
I have the pressure pro system and it works great. We have them on all of our trailer tires and the inside duals on our truck. I will be putting them on the outside dual next. Do to lack of clearance on our front tires will wait the right kind of extensions. Don't waste your money on the locks as they do not work that good.
I was just looking at the manual on the Doran PressurePro System (Doran PressurePro manual), and I don't see a way to install these sensors on the inside duals of a MH without taking the outside dual off.
Then once you get it installed, you have to take it off again to adjust the pressure.
to hertig--I just found this forum today and saw your comment about another tire monitoring system. I've been trying to research them and thought that there was just (Doran)PressurePro (RVTirePressure.com,AdvantagePressurePro.com,Doranmfg.com), Tire Sentry (tiresentry.com, griotsgarage.com) and SmarTire (SmarTire.com). Then I saw just one reference to Tire-Safe Guard (TireSafeGuard.com and the mother company's site Trikor.com).
Isearched through comments on rv.net and irv2.com and here is what I think so far--
PressurePro and TireSentry mount onto the valve stem and can cause problems (stem needs to be steel or the centrifugal force from the weight of the monitor damages the stem; must remove to add air and the o-ring can get damaged--people carry spares because the tire then leaks at the valve stem). PressurePro batteries are supposed to last 3 years, but aren't user replacable (must send them in). TireSentry batteries are user replacable, but I think someone said they're relatively heavy.
SmarTire is sold by Camping World and is the most expensive system. Sensor mounts on the hub before mounting tire and reports both pressure and temperature. I think the battery is supposed to last a really long time so you don't have to remove the tire to change it.
TireSafeGuard is the one I'm interested in, but can't find any user comments other than testimonials on their own site. Their system is priced by how many tires you're monitoring, is considerably cheaper than SmarTire, and is available with the sensors either (1) in a cap on the valve stem (with a bypass so you don't remove it to add air), or (2) built into a valve stem, or (3) on a strap-onto-the-hub. They won 2006 Product of the Year at SEMA in Vegas.
I would love to see anyone's input on these, especially Tire-Safe Guard (in the interior mount strap-on version.)
Smart Tire is the 'standard', but I have heard that people have problems with them. I'm pretty sure you can't replace the batteries, but have to replace the whole unit, for which you have to demount the tire to add insult to injury.
It sounds like TireSafeGuard is the one I was looking for. More versatile, but I don't have any idea how well they work. As I recall, the interior versions provide temperature as well, which is useful for spotting problems in the making, while the exterior version is a better design not requiring you to remove it to add air. Sounds like 'noone' really knows how well they work. Still, I would be tempted by the obviously better design and lower price. Perhaps I would start out slow, with external sensors and see how I like it, then move them to a toad and put internals on the motorhome.
Another thing I just found out about is 'Crossfire' for dual tires. This connects the 2 tires together so they always stay at the same pressure no matter what the road or temperature does. It provides a single point of applying air, an external High/OK/Low pressure indication, and safety valves in case of a blowout or leak in one tire. Can be got for under $60 per side. If one had this on their duals, it might be interesting to have a pressure/temperature monitor on only one of the 2 tires.
I came back to the forum because I realized that I was typing "hub" when I meant "wheel" in my previous post. Saw your reply. Of my 8 kids (Kathy-momx8, right?), 2 sons and a son-in-law are automotive engineers (Ford racing, GM racing, and Roush racing, as well as being racers themselves) so they have friends who know tires. They also all have motorhomes. So I passed the Tire-Safe Guard link on, and am waiting to see if they get any further information. If I learn anything more, I'll post. I'm holding off buying new tires until I decide, so I have to make a move soon if I'm going to do this.