When you print a large banner, it does not require as high res as regular photos. One reason is the viewing distance and another is printer’s printing resolutions. If it were on vinyl (PVC) substrates, it would typically be printed with 4 color solvent printer with 720 lpi. However, some sign shops run the printer much lower printing resolution than what printer is capable by prioritizing speed more than quality.
If you have an access for asking print shop prior to the project, that would be the best way. As my design work, I print photos on giant fabric banners and I used images from D700 (now I use D800E). However, in my opinion, RAW file from D700 (I am sure shop ask for TIFF) will be enough for commercial advertising banner.