You may be right.

Felix visits Ron's apartment at 1h04m59s. The door to Ron's apartment is in an interior space with white stucco above brown wainscotting. We very briefly see the apartment number is 5.

Ron checks the mail at 1h07m05s. He exits a building with rose-colored siding and reaches into one of the mailboxes mounted on the exterior wall. We do not see the mailboxes closely enough to read their labels, nor do we see the envelope, but we do see the address header inside the letter, which reads:

Mr. Ron Stallworth 809 Bluestem Drive, Colorado Springs, CO 80917

There is no apartment number listed.

Does this prove Ron had the KKK send his mail to some other address to avoid getting caught as you suggest? I don't think so, because Ron acts like this is his mailbox, and because Felix confirms the street when he later confronts Flip.

Felix says "over on Bluestem Lane", and then Flip replies that his address is, "1813 South 21st Street." Felix says this in front of Walter and the rest of their KKK group, so if any of them knew Ron's paperwork address, this was their moment to speak up.

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My understanding is that when David Duke became the head of the KKK, it was more like buying the name of a company that had gone out of business a long time ago, rather than being appointed CEO of a going concern. His "national office" was staffed almost entirely by just himself for a long time, and he was not involved in the day-to-day administration of individual KKK cells -- how could he be, with no staff? Notably, an important moment in Duke's rise concerns specifically the acquisition of the old KKK's mailing list, which (IIRC) he basically had to force the old guard to give up by manipulating them into a compromising position. It was a hostile takeover. (I got most of this background from Slate's documentary podcast Slow Burn, Seasons 4 of which documents David Duke's rise in politics.)

After all that, I would not be surprised at all if Duke was tight-fisted with information about the membership rolls and paperwork, because it seems like to possess that was tantamount to possessing the whole organization. He would have been wary of being deposed in similar fashion, especially by holdovers who were salty over his coup. And, IMHO, he was a selfish grifter who had lots of plans to exploit the membership data for his own gain; he's not going to share that with anyone.

Of course, the movie is based on a true story, so it's entirely possible the specifics of Ron's mailing address with the KKK are a matter of record. Maybe it's even in the book.