What I've seen a lot of guys around here do is to tow a trailer w/ an ATV on it for spraying larger complexes and what not. They fill the tank on the ATV from the truck, and then use the ATV to do the apps. This has become more popular. The other option would be to use a tank on a Z, or a new PErmagreen or similar spray unit.
If you fail to plan ..... you plan to fail.