I have tried this solution and it works great.

N-Level (MultiLevel) Bootstrap Responsive ASP.Net Menu control for Mobile Phone, Tablet and Desktop display

However, I have noticed one issue, if you add a scriptmanager to the page it breaks all dropdowns.

Do you have a method for fixing this

       <asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods = "true" EnableCdn="True">
           <Scripts>
               <%--To learn more about bundling scripts in ScriptManager see http://go.microsoft.com/fwlink/?LinkID=301884 --%>
               <%--Framework Scripts--%>
               <asp:ScriptReference Name="MsAjaxBundle" />
               <asp:ScriptReference Name="jquery" />
               <asp:ScriptReference Name="bootstrap" />
               <asp:ScriptReference Name="respond" />
               <asp:ScriptReference Name="WebForms.js" Assembly="System.Web" Path="~/Scripts/WebForms/WebForms.js" />
               <asp:ScriptReference Name="WebUIValidation.js" Assembly="System.Web" Path="~/Scripts/WebForms/WebUIValidation.js" />
               <asp:ScriptReference Name="MenuStandards.js" Assembly="System.Web" Path="~/Scripts/WebForms/MenuStandards.js" />
               <asp:ScriptReference Name="GridView.js" Assembly="System.Web" Path="~/Scripts/WebForms/GridView.js" />
               <asp:ScriptReference Name="DetailsView.js" Assembly="System.Web" Path="~/Scripts/WebForms/DetailsView.js" />
               <asp:ScriptReference Name="TreeView.js" Assembly="System.Web" Path="~/Scripts/WebForms/TreeView.js" />
               <asp:ScriptReference Name="WebParts.js" Assembly="System.Web" Path="~/Scripts/WebForms/WebParts.js" />
               <asp:ScriptReference Name="Focus.js" Assembly="System.Web" Path="~/Scripts/WebForms/Focus.js" />
               <asp:ScriptReference Name="WebFormsBundle" />
               <%--Site Scripts--%>
           </Scripts>
       </asp:ScriptManager>

RayKD