We are a mobile service business. Are vehicle payments, insurance, gas, and maintenance part of the cost of goods sold?

Unless you can directly relate the fuel purchase to any individual service invoice then I would lean towards none of what you listed as being COGS but instead as operating expenses. Here are some background links

http://www.business-literacy.com/financial-concepts/cost-of-goods-sold-cost-of-services/

https://blog.projectionhub.com/what-is-cost-of-goods-sold-for-a-service-business/

https://smallbusiness.chron.com/direct-indirect-cogs-service-industry-75032.html

you will see that one can find any answer one wants on the internet, and as with all tax related choices it is best to review with your own tax CPA