I believe that they fail because they are just cheap knock-offs of the ones mandated by the federal government for all vehicles under 10,000 pounds. The ones on most vehicles have a life span of 5-12 years and 100,000 miles and they very seldom give any problems at all. In fact, until the signal light goes on on your dash, most people do not even give them a second thought.
My experience with the RV TPMS systems shows they are inaccurate and have a battery life of 18-24 months. I use a hose coupled inflation system so I am inflating my duals and my front tires together at the same time. I still get TPMS pressure readings that often show 3-4 pounds of variance between a pair of tires that were connected together during inflation.
When TPMS systems were mandated for automobiles and light trucks, the cost was estimated to be less than $70 per vehicle, for the complete system. Naturally they were dealing with a much larger quantity but they also seem to be built at a much higher quality level than the units sold for RV's.