I doubt you will find a "manual" for your Ragen 21FB, just like I couldn't find a "manual" for our '09 WW FB2100, Tige 21V or Itasca Sunsetter. What you can find is manuals for each of the pieces of equipment the manufacturers installed in our RVs. We were looking for an '06 Ragen 21FB and ended up with an '09 WW FB2100, which is very similar. Fortunately the original owner kept all the manuals - generator, refrigerator, A/C, etc., but I've found them somewhat lacking. There is some good info on the appliance manufacturers websites and on forums, like this one. I use Google, a lot. As I find part no.s, procedures, drawings, pictures, PDFs, etc. for equipment on our trailer I save them away. I buy most of my parts from ebay, Amazon, etrailer.com and Walmart.com.
Our WW FB2100 has a push/pull switch in the power compartment which is used to disconnect the 12v converter from the 12v system in the trailer. The switch "behind the tongue" on your 21FB is probably used to disconnect the battery from the 12v system in the trailer. If the switch is exposed to road spray you might want to consider using a marine switch or relocate it to inside a compartment. The switch would be used to disconnect the battery when in storage without ground power to maintain a charge. Or to disconnect the battery from the 12v system if a cell is shorted and dragging down the converter output. Switches for both the battery and converter to disconnect them from the 12v system is nice when troubleshooting or modifying wiring.
2009 Weekend Warrior, FB2100, 23ft, 7.5k, Maxxis ST225/75R15, Dexter EZ Flex, Equalizer 10k