I use my Truck Systems Technologies 507 TPMS System on my RV, Tow Dolly and Camry rear tires (touching the ground) and it works great. It has alarmed twice when I have had slow leaks in the Camry tires due to road shrapnel, so I am very glad I have it.
Regarding your questions:
-The .54 oz. of extra weight (with these sensors) does not seem to be enough to cause any problem with the tire balance. I agree no one is in the vehicle but there is no odd tire wear with them installed. I take them off after a trip and let the internal factory TPMS work for normal use.
-The ď507 RV Cap SensorsĒ (which I have 14 of) have an anti-theft feature built in and Iíve never lost one
-These cap sensors can be used on rubber stems (some sensors require metal) and do not leak if installed properly, i.e. just snug and not super tight
-Batteries are user changeable and mine last two years as I remove the sensors from the tire when the RV is in storage for the winter
I hope this helps.