In most cases, we can just use the past participle if we need the adjective form of a verb, such as saying camino olvidado or coche abollado. When/why do we use lleno instead of llenado?
We say lleno rather than llenado because lleno is the original word from which the verb llenar derives, of which llenado is the passive participle. This comes all the way from Proto-Indo-European:
PIE *pl̥h₁nós (“full”) → Latin plēnus → Sp. lleno
So lleno exists as a simple word before the verb llenar, which is formed by derivation in the same way as other verbs, such as vaciar from vacío.
In contrast, the verb abollar exists before its participle abollado (there's no pre-existing simpler adjective *abollo), and olvidar exists before olvidado. (There's olvido, but it's a noun and appears to be a back-formation from the verb.)
Of course there's also some randomness involved in which words stay or leave, and which are used for which purpose. Spanish could very well have regularized this situation, dropping lleno and using llenado as an adjective, following what is done with most other verbs. It sounds uneconomical, but then today most people say búsqueda instead of busca for "search (n.)"; a derived longer word has replaced a shorter, more primitive one.
I am new in this site but I will try my best.
If we take a look to the grammar, llenado is different from the other two words. While abollado and olvidado are the participle of the verb and we can use it as an adjective, the word llenado it is not an adjective but a noun. Then, as all the nouns, they can be the subject of the sentence. If we want to use an article to say that is full, we have the word lleno.
The petrol tank is full // El depósito de gasolina esta lleno.
The filling of the petrol tank cost 40€ // El llenado del depósito de gasolina costó 40€