One thing to be aware of-- you said OnSong lets you choose Bank Select MSB 1-128 and Bank Select LSB 1-128, and I assume it also has Program Change 1-128.
The numeric values used in the actual MIDI messages are always 0-127, because any value over 127 (i.e., 128-255) will have bit 7 of the data byte set to "on" (or binary "1"), and MIDI uses bit 7 as a status flag except in very specific exceptions.
However, the MMA (or MIDI Manufacturers Association) frequently lists the MIDI Program Numbers as 1-128, even though they are always encoded in MIDI messages as 0-127.
There is no reliable consistency in printed material as far as whether the Program Numbers are listed as 0-127 or as 1-128. For example, Yamaha normally lists their Program Numbers as 1-128, although the data lists for older Yamaha models may use 0-127; whereas Casio normally lists their Program Numbers as 0-127.
Since there's no consistency between manufacturers-- or even between all models past and present that have been produced by a given manufacturer-- you just have to check the data list for a given keyboard to determine which numbering scheme it uses. To do this, look at the Program Number (or Program Change value) which is listed for the Acoustic Grand Piano sound. If it's listed as Program 0 (not Voice 0 or Tone 0, since the Voice Number or Tone Number is not the same thing as the Program Number), then the data list is showing Program Change values of 0-127. But if it's listed as Program 1, then the data list is probably showing Program Change values of 1-128, although you should verify this by scanning the rest of the sounds to see if any of them are listed with Program Numbers of 0 or 128. Basically, if any of them use a Program Number of 0 then the data list is using the 0-127 numbering scheme; whereas if any of them use a Program Number of 128 then the data list is using the 1-128 numbering scheme.
Bank Select MSB and LSB values are almost always shown in keyboard data lists in accordance with the 0-127 numbering scheme-- at least, I don't think I've ever seen a keyboard data list that used the 1-128 numbering scheme for the Bank Select MSB and LSB.
MIDI software is another story. Some software uses 0-127 for the Bank Select MSB and LSB as well as for the Program Change, whereas other software uses 1-128 for these. I don't remember ever seeing any software that uses 0-127 for Bank Select and 1-128 for Program Change, but I wouldn't be surprised if there were. Note that some software might let you choose which numbering scheme you want to use, so that you can set it to match whichever numbering scheme(s) your keyboard manufacturer uses in their data list. Also, note that some software may show the Bank Select as a single value ranging either from 0-16383 or from 1-16384 (i.e., where the 7-bit MSB and LSB values have been combined into a single 14-bit Bank Number).
The lack of any consistency is absolutely crazy in my opinion, because it's almost like the people who make MIDI hardware and software are trying to drive us mad-- or perhaps trying to ensure that the only way you can get your MIDI hardware and software set up the way you want it to work is by hiring some outrageously-expensive high-falutin "MMA-Certified MIDI Technical Specialist" to do it for you.
Anyway, I can see by the RD-300NX Data List that the Program Change values are listed according to the 1-128 numbering scheme (see the PC values listed for "Concert Grand" on page 3, and for "Bandoneon" on page 4), whereas the Bank Select MSB and LSB values are listed according to the 0-127 numbering scheme.
This means that if OnSong is asking for Bank Select MSB and LSB values of 1-128, you'll need to add 1 to the MSB and LSB values which are shown in the RD-300NX Data List. On the other hand, if OnSong is also asking for Program Change values of 1-128, you can just use the PC values which are shown in the RD-300NX Data List without having to adjust them.