I looked a for a forum on rhodies but, apparently there isn't one.

Several of my old rhodies, I'm talking 30+ years, are dying. It starts at the distal end of the branch and works its way back to the main trunk.

There have been no introductions of other plants or soil but around them.

https://www.rhododendron.org/i...

https://www.rhododendron.org/v...

It depends on the variety. Catawba rhododendrons can live over 100 years. Look for environmental factors, root rot, weevils or fungal diseases.

Hmmm...
Along with the environmental factors mentioned... Maybe a soil test...

They prefer acid soil... Some concrete work nearby can have negative impact... And... Apparently nut trees can affect adversely...

Possibly climate change variables?
Difficult to grow in this area... but some do grow for a while...

30 years sounds like a long time...

I've had success propagating from cuttings and layering.