Cambodia where? if Siem Reap and including Angkor... please read on for my thoughts, else not applicable hehe.
Only two lenses and a tripod are the necessary for my Siem Reap trip.
* 10-22mm for temples withins.
* 24-70mm for landscapes and all sorts.
they are on FF camera.
So probably, i feel your 18-55mm will be the most used lens, and hardly the 50-200mm (or 55-250mm), and you might want to save buying a flashgun for this trip which hardly serves much purpose since i assumed you got an onboard flash for fill-in?
An UWA lens should be better choice, but again, i feel you shouldn't use your vacation (unless you have the luxury of time) as a practise session for the UWA which requires some understanding and practises for it to shine.
In summary, just make do with what you have, enjoy the vacation, and not thinking what gears to supplement for the vacation cuz you need time to master those gears.