You can move some apps from internal storage to SDCard (not the physical external SDCard). Which means from the default location to NAND, if I've remembered my partitions the correct way round.
I'd move a couple of apps over, consider clearing the cache in XBMC if it is too large, then installing ZDBox. Once installed, disable the battery monitor and data usage services, then click on system cleaner. You can also use this app to stack up moving apps to SDCard.
See here -
http://droidboxforums.com/threads/tweaking-apps.102/ - for ZDBox link, and here -
https://droidbox.co.uk/blog/the-advert-stated-4-or-8-16-gb-rom-i-see-a-lot-less-why/ - for SDCard, Internal Storage, NAND and external SD card explanation.
If, after all of that, you still don't seem to have much free space, you could consider a factory reset via the Recovery Menu, see here -
https://droidbox.co.uk/blog/how-to-access-your-droidboxs-recovery-menu/ - not the Android Settings factory reset though, that doesn't seem to do the job on RockChip based devices. Obviously make sure you've backed up anything important first.