Purchased my Cat powered coach 4 1/2 years ago ; 52,000 then 79,000 now; only issue was a rather persistent oil leak , that I hopefully cured with genuine Cat parts this winter. Aftermarket parts failed consistently every 3,000 miles.
Haven't heard about head gasket issues on the 3126B , like I have.
Maybe a browse through the Caterpillar section of the " Powertrain " forums would answer that question.