There is a spreadsheet file that has been available online from various websites that you may find useful in calculating the combined weights of your tow vehicle and trailer. I have altered the original file to make it a little more user friendly and uploaded it to this site (see link below).
The spreadsheet allows you to enter vehicle weight ratings (tow vehicle and trailer), tow vehicle weights (dry weight, people, cargo, fuel), and trailer weights (dry weight, options, battery, propane, water, and stuff loaded inside the trailer). Calculated combined vehicle weights are shown near the bottom of the spreadsheet.
I didn’t create the spreadsheet and haven’t been able to find out who did. It appears on several websites when you search for “towing capacity worksheet”.
Download the file by selecting the link below:
In addition to the above, there is also a GVWR and Payload Calculator. There is a spreadsheet file that can be downloaded here or you can use the online calculator here. There also is a video on YouTube you can watch to learn more.