Silverleaf responded, here is what they said.
"Your coach uses the SmarTire system. I believe you can still get SmarTire sensors. I'd recommend calling Bob Dickman Tire for more sensors - 541-998-3319.
If you prefer to use a different tire monitoring system, we also support the Pressure Pro system. Their sensors screw onto the valve stem rather than strapping to the rim like the smartire. You can purchase pressure pro systems through us. The base kit (KT801) that includes receiver, antenna, repeater, and cables is $423. Sensors are $50 each. You would also need a software update to your VMS400 (dash unit)."
So to upgrade to a more current system would be 723 dollars not counting labor to install or the software update. I have one smarttire out, and I think replacing the others might be a good idea when we put in new tires. If the batteries can be changed I will do that instead of replacing the units.
Monty & Janet - 2007 Alpine APEX 40 MDTS
S/N - 75715 - Retired - Master Certified RV Tech