Rare mushrooms add 50 food per. Don't decay. Easy to get from megachelons. Rare flower work as well. Beware they starve extremely fast. So only food with high enough food value will keep them alive. Aka not the seeds u get from megachelons. That will cause them to starve faster than they can eat.
I took my argentavis and a doedicurus. Gathered as much stone as the doedi could handle (a few thousand), then dropped stacks of stone (from my inventory) a few at a time (short spoil time, be aware). A level 145 only took a few minutes this way.