You've got to sell one whole Hell of a lot of plate lunches to pay for a $50-200K truck and all the ancillary expenses and problems and if you don't live in the tropics your market tends to shrink in winter. Might be cheaper, easier, more manageable, more profitable, and more fun to do some "niche" catering, even if you have to rent a commercial kitchen space and/or buy a few more toys tools.
Jus' sayin'.