Hi Whippetguy,
Welcome to the Heartland Owners Forum.
Mallards are a derivative of another Heartland trailer, but are built exclusively for Camping World and to Camping World specifications. So it's possible the original Heartland model may come with a ladder, but Camping World specified the Mallard without it.
When trailers are built with ladders, there's reinforcement built into the roof where the ladder mounts. If the trailer doesn't come with a ladder, it probably doesn't have the reinforcement and won't support a roof-mounted ladder, even if it looks like it will fit.
Others have purchased telescoping or folding ladders. The telescoping models fit into a very small footprint and can be stored in your pass through basement storage.