Rhododendrons can live for many decades - there are many in my garden that are well over 50 years old!! Yours is just suffering from a lack of sunlight. Can you thin or prune out some of the tree canopy to allow better light penetration? Pruning could help the plant to fill in a bit by stimulating dormant buds but this will be far more successful if the plant received better light.

There's no way I can prune out the canopy - huge huge oak trees that are already limbed up but still provide the lovely shade - but not lovely for my rhodie. Might you be able to suggest something else that I could put there in dappled shade? I do love the color of the rhodie though - and must also add that all my azaleas in the very same area bloom profusely.

The first thing that comes to mind is, as gardengal suggests, too little light. There is, however, something of a paradox in what you describe. In general, azaleas, if they are to bloom well, need more sun than rhododendrons. High dappled shade from limbed up oaks ought to provide excellent conditions for this and any other rhododendron. The leggy growth habit may be a characteristic of this particular variety. I'd try pruning back a small number of the inner branches to see if new growth sprouts well. If it does, more pruning. can be done in the future.

Another possibility: if there is lots of direct sun or especially strong light as there appears to be to the right in the photo, all growth may have become concentrated in that direction, leaving the more shaded portions bare.

Rhodies respond really well to hard pruning. My landscape is 40 years old. I watched most of my Rhodies begin to look like yours, so I pruned timidly, and it helped, but the plants needed to be smaller as they aged the landscape. I pruned the tall lanky plants by 1/3 twice over a period of three years. They came out full and gorgeous. It's like they got a facelift. If you time the pruning right you will even enjoy blooms next year. Youtube has videos on pruning rhodies this way. Good luck.