ISA certified arborist:

https://wwv.isa-arbor.com/membership/benefitstools/findtcs

My state extension service is quick, inexpensive and good. I figured out that most tree services sent their samples to the state lab and charged for for it and didn't always report back the results.

What's the mushroom? Mushrooms vary significantly from ones that have no association with a tree, those that are symbiotic and those that only grow on dying trees.

Mushrooms actively growing on the trunk or branches of a live tree are most generally indicative of some sort of interior rot. The actual decline of the tree itself may not be obvious and they can often live for many years with interior or heart rot but it is a fatal condition. That is probably what your power company was alluding to.

As Bill noted, you don't want a "tree service" - you want a certified ISA arborist. Tree services (unless an ISA service) are not qualified to assess tree health. But a certified ISA arborist is and needs no extension service to make a valid diagnosis.