I am facing the same problem, but I guess it is more of a question for the output style.

There is an API for BibTex data for each RFC using URIs like so

https://datatracker.ietf.org/doc/rfc3986/bibtex/

It includes a field `howpublished` which contains "RFC xxxx" as a string.

In Zotero, it seems to map to the "Report Number" field.

So one would simply need to modify the output style to add that field before the title of a report in the list of references.

Personally, I would also prefer to have the URI/URL of the RFC in the reference entry.

Here is what the API returns for RFC3986 btw.

@misc{rfc3986,
series = {Request for Comments},
number = 3986,
howpublished = {RFC 3986},
publisher = {RFC Editor},
doi = {10.17487/RFC3986},
url = {https://www.rfc-editor.org/info/rfc3986},
author = {Tim Berners-Lee and Roy T. Fielding and Larry M Masinter},
title = {{Uniform Resource Identifier (URI): Generic Syntax}},
pagetotal = 61,
year = 2005,
month = jan,
abstract = {A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. {[}STANDARDS-TRACK{]}},
}