Trailer towing and manual transmissions don't mix well. Most manufacturers make an automatic transmission a mandatory part of their heavy towing package, even on vehicles on which a manual is available. The reason for that is that getting started with a heavy load, especially uphill, is murder on the clutch. To modify a vehicle for heavy trailer towing, the factory has to either give it a heavy-duty clutch or an automatic transmission with a good transmission cooler. Few manufacturers follow the "heavy duty clutch" route.