I used to do it in the van - became a burden actually - estimating and billing from the computer in the van - I like emailing a quote and getting the response in writing from the customer in the form of a reply email. For small jobs that is a legal enough document for us to do the work.
It also gives me time to think through the job and not forget anything and to check stock prices.
The paperwork from the day comes to the office and it all gets entered into the computer in the office - esitmates, jobs performed, checks in the mail, checks from service work that day -
For a one man shop -it may be easier to have the computer and printer in the truck though - it all comes down to how you like to run things.