I'm not aware of any requirement to have metal valve stems for the TPMS. At the RV dealership where I work, every system we have installed, included Pressure Pro and TireMinder, have been on rubber stems. No issues that I am aware of. Or is this just a personal preference you have?
Casey
1995 K1500 Tahoe 2 door, 6.5LTD, 4L80E, NP241, 3.42's, 285/75R16 BFG K02's; 1997 506 block; Kennedy OPS harness, gauges, Quick Heat plugs, and TD-Max chip; Dtech FSD on FSD Cooler; vacuum pump deleted, HX35 turbo, Turbo Master, 3.5" Kennedy exhaust, F code intake; dual t/stats, HO water pump, Champion radiator; Racor fuel filter