Disk Utility says Bootcamp partition has storage left but in Windows, the C: disk says there is no volume left? I partitioned 42GB for bootcamp and the windows system is about 17GB, I have one game on it plus Steam which combined with system files makes up to 34.1, and disk utility says I have 8.6GB free but when I'm actually in Windows, it says C: disk is almost full. Where did this 8GB go?
Very likely pagefile &amp;/or hiberfile, each of which can be the same size, or even larger than, your total RAM size. You can remove/disable the hiberfile but not the pagefile.
Having only 8GB free space on a drive is actually critically low anyway. You ought to have at least 10% even on a large drive/boot volume. On a tiny drive unfortunately you really need even more. 15 - 20 GB would be the minimum free space I'd recommend on a drive that size.
How-to-Geek has an article on how to disable hiberfile - What Is hiberfil.sys and How Do I Delete It? which I won't précis here as it's really a Windows issue not Mac. Basically, you stop Windows from allowing Hibernate then you can delete the file.
There are many disk/map/storage apps for Windows so you can see what may be using more space than you'd like -  I don't use Windows enough to know which might be best suited. This site lists several of them. Hongkiat - 10 Best Disk Space Analyzer for Windows 10
It is possible to customise your pagefile size, but I would recommend against this, &amp; certainly against reducing it below your RAM size, as it can cause crashing/memory issues.