Here is a fix that I found that worked. Upgrade the report to be version 10 for the report viewer, so using all version 10 assemblies and references.
then in the web.config add the following to system.webServer:
<handlers>
<add name="ReportViewerWebControlHandler" preCondition="integratedMode" verb="*" path="Reserved.ReportViewerWebControl.axd"
type="Microsoft.Reporting.WebForms.HttpHandler, Microsoft.ReportViewer.WebForms, Version=10.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />
</handlers>
After doing this, my report worked.