Do you tow a vehicle behind your RV? If so, it is recommended that you use the 507 cap sensors on the tow vehicle. The TST Tow Car Pack includes four 507 cap sensors that work with the 507 systems. Even if you use the flow thru sensors on your RV, it is recommended that you use the cap sensors on your tow vehicle because the cap sensors have a lower profile and work best with shallow wheel wells.