If you have your truck and trailers already in the budget as equipment items, you can use the below method to combine the costs to easily create a truck+trailer combined equipment item.
First, import the truck as normal, record on a note pad the cost per hour:
Then, import the trailer (which should be a grouped item in your budget) as normal. Use the calculator to come up with the cost for the trailer as normal:
Now, outside of LMN add the price per hour of the truck and the trailer together. In this case we have the truck at $11.64 and the trailer at $2.25 for a total cost per hour of 13.89.
In the trailer item, put the new cost in the cost box, change the name to show it is the truck and trailer combined and add in details to explain where the cost came from:
And now you have a combined truck and trailer item with accurate costs from your budget and minimal math.