I haven't seen the RCP calculator but does it take into account the fact that 18 states are winner takes all? This amounts to nearly 1,000 of 2,472 delegates.
If Trump wins a mid-30s percentage of the proportional delegates (500, say), he needs 737 of the winner-takes-all states. That looks very doable to me as he is leading everywhere I am looking.
This also undercounts Trump's likely haul in the proportional states as many disqualify candidates with a small share of the vote from receiving delegates, meaning those who qualify end up with more.