Hey! Welcome to the Forums!
This is the best tool that I have been able to find to work through the math aspects of towing:
Pop-the-top - Dedicated to Popup Camping and Hunting
In the left-hand pane, in the middle of the page, you'll see a hyperlink to "Towing Capacities" When you select it you will be prompted to open an Excel worksheet. If you don't have a spreadsheet program I highly recommend Apache's OpenOffice - it is free!
You'll find that manufacturers don't typically report all of the information that you'll need to fill in the sheet. The most difficult data to find usually is the base weight of the tow vehicle. I found that I typically had to find an electronic copy of the manual online. Inside the manual, the empty, dry-weight of the base model should be available. Unfortunately you will most likely have to estimate the weight of any options added above and beyond the most basic configuration.