Can I step down a NEMA L15-30R 90A 250v outlet to 110v?
No. You can't use this for 120V power. The NEMA L15-30 receptacle is intended for 3-phase power applications. Most likely, it is a 208V three phase power outlet lacking neutral. Because there is no neutral, there is no easy or reliably safe way to get 120V power from it. Even if there were a neutral, it would be questionable whether it would be a good idea to apply a load to only 1 phase. There could be circuitry upstream designed to trip on unbalanced loading.

If you want to confirm that it really is a three-phase outlet, simply measure the voltages from phase to phase. This involves sticking voltmeter probes into the receptacle. There is a shock hazard doing this kind of thing, especially if you are using a cheap multi-meter. If you don't feel comfortable doing it, then don't do it. Please don't blame me if you get shocked.