using System;
using System.ComponentModel;
using System.Windows.Forms;
using System.Runtime.InteropServices;
 
public class RichTextBox5 : RichTextBox {
    private static IntPtr moduleHandle;
 
    protected override CreateParams CreateParams {
        get {
            if (moduleHandle == IntPtr.Zero) {
                moduleHandle = LoadLibrary("msftedit.dll");
                if ((long)moduleHandle < 0x20) throw new Win32Exception(Marshal.GetLastWin32Error(), "Could not load Msftedit.dll");
            }
            CreateParams createParams = base.CreateParams;
            createParams.ClassName = "RichEdit50W";
            if (this.Multiline) {
                if (((this.ScrollBars & RichTextBoxScrollBars.Horizontal) != RichTextBoxScrollBars.None) && !base.WordWrap) {
                    createParams.Style |= 0x100000;
                    if ((this.ScrollBars & ((RichTextBoxScrollBars)0x10)) != RichTextBoxScrollBars.None) {
                        createParams.Style |= 0x2000;
                    }
                }
                if ((this.ScrollBars & RichTextBoxScrollBars.Vertical) != RichTextBoxScrollBars.None) {
                    createParams.Style |= 0x200000;
                    if ((this.ScrollBars & ((RichTextBoxScrollBars)0x10)) != RichTextBoxScrollBars.None) {
                        createParams.Style |= 0x2000;
                    }
                }
            }
            if ((BorderStyle.FixedSingle == base.BorderStyle) && ((createParams.Style & 0x800000) != 0)) {
                createParams.Style &= -8388609;
                createParams.ExStyle |= 0x200;
            }
            return createParams;
        }
    }
 
    [DllImport("kernel32.dll", SetLastError = true, CharSet = CharSet.Auto)]
    private static extern IntPtr LoadLibrary(string path);
 
    //ScrollToCaret-Speicherzugriffsfehler beheben
    [DllImport("user32.dll", CharSet = CharSet.Auto)]
    private static extern int SendMessage(IntPtr hWnd, int wMsg, IntPtr wParam, IntPtr lParam);
    private const int WM_VSCROLL = 277;
    private const int SB_PAGEBOTTOM = 7;
 
    public void ScrollToBottom()
    {
        SendMessage(this.Handle, WM_VSCROLL, (IntPtr)SB_PAGEBOTTOM, IntPtr.Zero);
    }
 
}

Quellen: RichTextBox row height
Inconsistent Results with RichTextBox ScrollToCaret