1/03/20: Edited to provide examples as suggested by jlawler.
It's a pretty straight forward question.
Here are the definitions from SIL:
Definition: A relative pronoun is a pronoun that
- marks a relative clause
- functions grammatically within the relative clause, and
- is coreferential to the word modified by the relative clause.
Definition: A relative adverb is a pro-adverb that marks a relative clause.
Definition: A relative clause is a clause which describes the referent of a head noun or pronoun. It often restricts the reference of the head noun or pronoun.
Here are links to defintions and discussion at YourDictionary.com
It seems that the definition of a relative adverb is subsumed in the definition of a relative pronoun. So, I'm looking for examples where this distinction becomes clear and its usefulness is established.
Ok, so the distinction seems pretty straightforward with most of the words that are used to join relative clauses but not with where, why and when.
The church where we were married has burned down.
Those times when she is off her meds cause great concern.
The reason why he left you makes no sense.
In these sentences, where, when and why mark an embedded relative clause that adds information (locative, explicative, descriptive) to the subject phrase without changing its basic meaning. They are conjunctive and have a clear pronomial relation to their antecedents, the church, those times, and the reason. They are adverbial only in as much as they modify the verb in the relative clause. These are clearly relative pronouns, but I would not call them relative adverbs because they are not adverbial in their relation with the main verb, and because even if adverbs can modify nouns, these adverbs haven't changed the interpretation of the subjects; the church is just a church, and so on. These words refer to their antecedents and join them to a descriptive clause, making the relation they establish adjectival, not adverbial. They are also adjuncts.
Within the relative clause, they're just adverbs, as they are in questions like, Why did he leave you? and Where were you married? and so on. But they do seem to be taking the role of a preposition, as some commenters have suggested, where the clause is functioning as an object. But do prepositions really have a pronomial feature? Not so sure about where, why and when as prepositions, and I'm hard pressed to find a dictionary that lists preposition as a (PoS) for these words. But I get it, so that's another way of looking at these relations.
So anyway, by these definitions - where, why and when in the above sentences are not relative adverbs.
In the following sentences they seem adverbial but do not introduce a relative clause, strictly speaking - they are predicate adjuncts.
They found the ball where the dog had dropped it.
I love the church where we were married.
I am concerned about those times when she is off her meds.
Your cheating is the reason why he left you.
These relations seem to be with the the whole predicate phrase, found the ball, and so on, in as much as the clause does not have and independent relation with the main verb or its object complement. So, I don't think where qualifies as a relative adverb, nor is this a relative clause because it is not strictly about the ball (maybe).
Here are the examples from YourDictionary.com for relative adverbs:
Gone are the days when I could stay up all night.
This is an inverted copular clause - Gone is a past participle adjective, The days when I could stay up all night are gone, Gone is a subject complement - when is either a relative pronoun, or it's functioning like a prepostion (in which) with a clausal object. A similar argument could be made for the following examples.
The 50s were a time when the family unit was largely intact.
That is the year when we got married.
We danced [by the table] [where (at which) we could see the view].
The bolded text is the core sentence, the phrases in brackets are independent adjuncts. Where is locating the action so it is cleary adverbial in relation to the main verb, but it is also describing the location of the subject, and the table, it takes the table as its pronomial referent, and it is conjunctive/ prepositional. It has all these properties, so why narrowly insist that it is a relative adverb?
Similar arguments could be made for the following examples:
This is the coffee shop where we'll find the best cup o' joe.
This is the garden where they took their photos.
Her mass of library of books is the reason why she's so well-spoken.
The main verb is copular, so similar arguments (as above) apply. The relative clause is an adjectival adjunct, and why is pronomial, conjunctive and prepositional (for which).
In the following examples I see no adverbial relation to the main verb, only a pronomial relation to its object complement:
Can you provide more information why this conclusion is valid?
I have no idea why he called.
As far as I can tell, in every case where one of these words is described as a relative adverb it could also, and perhaps more validly, be described as a relative pronoun. Is it called an adverb just because that's how these words are usually identified? SIL calls the relative adverb a pro-adverb which is just another way of saying that it's not really and adverb. I have a feeling that the term relative adverb is a bit bogus, it seems to be a distiction without a difference, unless someone can provide an example where one of these words is strictly and solely in an adverbial relation to the main verb and marking a relative clause, or at least a case where it makes a useful distinction. I can't think of or find one, and that's just me and I'm not an expert, but it seems impossible.
To answer my own question, I think it may always be the case with where, why and when that these relative adverbs can also be validly described as a relative pronoun. But I'm hoping someone can prove me wrong, otherwise, what's the use of this term?