There's a few different considerations at play here. Since the numbers come from the Russian Ministry of Defense and we quote RT citing the numbers, we can say they are trustworthy in the sense that we point out where our numbers come from.
However, I can expect that the MoD is fudging the numbers a little bit. We would never know though, we can only speculate. I'm not sure more accurate figures are available; those "think tanks" that try to tally the numbers don't always give better figures.
Ultimately we've done our part. We could always add a second source alongside the MOD figure, but we clearly stipulate where we got the numbers from; in this sense we don't even take a side on the figures, we just say "here's what the MoD says the casualties are"