The DRUMS = CHANNEL 10 thing is really a General MIDI Level 1 (GM, or GM1) requirement, since the GM1 specifications didn't have Bank Select messages. So in GM1 you're limited to 128 Program Change numbers for the "melodic" instruments-- i.e., the ones where each Note event plays a different pitch. And since there is no Program Change number for the drum kit, there's no way to select between a melodic instrument and the drum kit, hence the drum kit is exclusively assigned to channel 10, such that the drum kit cannot be played on any of the other channels, and none of the melodic instruments can be played on channel 10.
However, that limitation and its corresponding requirement were removed in General MIDI Level 2 (GM2), as well as in manufacturer-specific extensions of General MIDI Level 1 such as GS (Roland) and XG (Yamaha), thanks to the introduction of the Bank Select MSB and Bank Select LSB events, coupled with multiple drum kits that can be selected just like the melodic instruments by issuing a specific combination of Bank Select and Program Change values. Drums are still frequently used on channel 10, and are often used on an additional channel as well (typically 9 or 11, I think), but that's now a recommendation rather than a requirement, since it's now possible to select a melodic instrument on channel 10 or a drum kit on any channel.
I'm not that familiar with Roland's GS extension of MIDI, but I know that Yamaha's XG extension of MIDI has some XG System Exclusive messages which can be used to set the voice mode (melodic or kit) for each of the keyboard's multitimbral parts and their associated MIDI channels. In XG the default is for channels 9 and 10 to be set to kit mode, but that can be changed using the proper XG SysEx messages. Also, I'm not sure, but I think that incoming MIDI messages will automatically override those defaults, since the keyboard's multitimbral parts and the MIDI channels are actually independent of each other (they do conform with each other on a one-to-one basis by default, but the assignment of a particular multitimbral part to a particular MIDI In or MIDI Out channel can be overridden as desired). Roland's GS extension may have something similar to that.
Anyway, refer to your RD-300GX documentation to see which Bank Select and Program Change numbers are used for the particular drum kit(s) you want to use, then make sure you send those values in a sequence of Bank Select MSB, Bank Select LSB, and Program Change messages on whichever channel(s) you want to use the drum kit(s) on.
Note that if you don't send any Bank Select/Program Change messages on a particular MIDI channel, and haven't already set that channel to a specific sound, the receiving hardware or software will usually default to Bank 0/Program 0, which is typically the Acoustic Grand Piano-- but it might be something else depending on the hardware or software, or which mode the hardware is in, etc.
Of course, if the hardware or software is limited to GM1-compatibility, then you'll get drums on channel 10 regardless of what Bank/Program you try to select, and likewise you'll get one of the 128 GM1 programs on the other channels even if you try to select a kit voice (percussion kit or sound effects kit).
If you send a Program Change without an accompanying Bank Select then the receiver will usually default to Bank 0-- unless a different bank has already been chosen by a previous Bank Select message or by setting the receiver's mode (e.g., to switch to a special bank of user-defined patches).
