I have the same adapter on my Nikon D600. It's not too good at focusing at over 200mm, but at 600mm, (double your 300) actually would benefit from manual focus on a tripod.

I picked up a clean D600 on Ebay a while back. I too am retired & on a fixed income. Many people are afraid of the D600 because of the oil issue on the shetter, but most of them have been fixed, but Nikon has said it will fix the issue free if it happens. I love my D600 & I've gotten a few film era lenses for it very reasonably. Have fun!

It appears there might be an issue with your lens. Is it new? One more thing to try...lock the aperture ring to 32.

Check that the af switch on the body is turned on. Do you have any other "D" lenses...do they autofocus? If the camera is set up for back focus, it will only focus with the "AE-L / AF-L" Button. Push that by itself & see what happens, then push the shutter release. I have a Nikon 80-200 2.8 lens I'd swap for your 300.

That lens should work fine on any Nikon with the focus motor built in to the body.

Hi, Diane. I'm also on a fixed income & just moved to a full frame. I went with the D600. It's older, but it works great & fairly inexpensive on Ebay. They're plentiful & lenses are inexpensive as well. I also use a D7100 so lenses are interchangeable. Best of both worlds. Nikon says the D600 is good for 150000 clicks, so should last for quite a while. Enjoy!
Bob