The English language that was used in the KJV was already obsolete when the KJV was published in 1611. It was translated from what I understand from medieval Jewish texts and was full of errors in translations. I do not know if that is still the case in the RKJV but, having read many different versions of the Bible both Catholic and Protestant I find that the Catholic versions are the more accurate translation.
Much of the KJV was based on Erasmus’s first printed translation of the NT into Greek and Latin. In a way it is legitimate to say that Erasmus “synchronized” or “unified” the Greek and the Latin traditions of the New Testament by producing an updated version of either simultaneously. Both being part of canonical tradition, he clearly found it necessary to ensure that both were actually presenting the same content. In modern terminology, he made the two traditions “compatible”. This is clearly evidenced by the fact that his Greek text is not just the basis for his Latin translation, but also the other way round: there are numerous instances where he edits the Greek text to reflect his Latin version. For instance, since the last six verses of Revelation were missing from his Greek manuscript, Erasmus translated the Vulgate’s text back into Greek. Erasmus also translated the Latin text into Greek wherever he found that the Greek text and the accompanying commentaries were mixed up, or where he simply preferred the Vulgate’s reading to the Greek text.[35]
Metzger, The Text of the New Testament, pp. 99–100; Kurt Aland – Barbara Aland, The Text of the New Testament. An Introduction to the Critical Editions and to the Theory and Practice of Modern Textual Criticism,Translated by Erroll F. Rhodes. Grand Rapids: Eerdmans, 1987. Second edition, revised and enlarged, 1989, p. 4
Since neither his Greek nor Latin translations were based on accurate interpretation of his oldest texts, and in some cases, his original source material was lacking, so he borrowed from his other translation, the final products were, in effect, edited to fit his purposes. That is why critics have a low regard for the accuracy of the KJV.