YOU don't budget running a truck. You really can't.
You have three different type of costs, two are fixed costs, one is reoccurring and the other is annual. then you have the killer - variable costs, that is fuel and things like tolls.
Your revenue isn't fixed, it will also be variable but bills have to be paid and this is where people get into trouble, they ignore the needs of the bills and ignore maintenance to pay themselves.
Speaking of maintenance, don't assume you are going to be able to fix a truck somewhere you're stuck at, even though you are a mechanic. Get used to the idea of paying someone and pay them.
I would save at least three months of down time, not a month.
AND pay yourself LAST, not first.