Running non-stop with no breaks, bodies ready to go, matching the efficiency of the most optimal stationary crematories, working around the clock, a single truck could burn 12 bodies using 336 gallons of fuel (per https://www.lng2019.com/how-much-natural-gas-is-used-to-crem...) and generating 6.6 metric tons of CO2. For this to even make sense politically, there would have to be a number of bodies so large that burning it made more sense than anything else, even with how costly and obvious it would be to observers.
I'm open to your guesses, but I'm going to say one thousand innocents in a single place would be sufficiently atrocious. I say this because numbers like 100-200 are thrown around fairly casually when reporting on Ukraine and other locations, so I'm going up one magnitude from that. So 84 trucks working as described could finish this job in one day, releasing 5,500 metric tons of CO2 and consuming 2300+ gallons of fuel to do so. I leave it to the reader to determine whether it's plausible for a military in the midst of a very difficult war to dedicate the human and supply resources necessary to conduct such an operation (at peak efficiency, as described).
At the very least, the movement and fueling of 84 such trucks (or even 30 if we're spending a week doing this) should be observable. We shouldn't have to rely on video from 2013 to make these claims. The video most often shown as proof of Russia's mobile crematories is one of a mobile incinerator, e.g. for trash which can fully combust in minutes.