I found this searching... https://www.gsowners.com/forums/show...61&postcount=4
The answers to your questions will depend on the model year and the layout. In my 5211, I have one sofa and a dinette. There is storage under the dinette seats. Under the sofa is where the fresh water tank, water pump, etc. is, so there is limited extra space for stuff.
If there were two sofas, there would likely be storage for the bedding/pillows and more that you mentioned beneath the curbside one. Good luck in your search.