Plain white vans are wasted advertising, you're right to think about it. We run an HVAC company with four trucks and did fleet branding with SmartSwans
https://smartswans.com/vehicle-wraps-branding/ - partial wraps with logo, phone number, a short service list and a QR code on the back. They were big on keeping it readable: large phone number, short text, contrast that works at a distance. Consistent design across all vans makes us look bigger than we are. We genuinely get calls from people who saw a truck in traffic. They also have a guide on their site about what to put on work vehicles, worth reading before you design. Budget for partial wraps if full wraps are too much.
