OTG is still a bit immature, and you have a nasty little tablet (I have a not dissimilar one for diags work), so you're in the land of the gods as to whether or not you can even get OTG to work, let alone OTG charging and media card simultaneously.
Mrs TB and I have gayPads that I exclusively use for going away (she uses hers more often). A 4.5hr flight will take the battery down to about 80% when using headphones, just to give you some comparison between the low end nasty shite and top end stuff.... ....helped of course be Apple's walled garden.
The said, not that I use for video, but none of my Windows tablets kill the battery in sub 2hrs.
Try uninstalling Kodi (an absolute resource hog that will constantly rape the CPU, hence battery), and using a decent video player, see if that improves things. Ideally a player that can exploit any hardware decoder built into the CPU/GPU. Kodi is shit. It always was shit in its XBMC days, and it will always be shit. It has one purpose in life, to allow people, otherwise too stupid, to do things we can't discuss on here.