There are a number of small companies that custom-manufacture farm machinery adaptive devices, including man-lifts to access large trucks. Among them are Life Essentials, of Brookston, IN (by phone @ 765-742-6707 or on the Web @ www.lifeessentialsweb.com) and John Hancock, of Lexington, KY (by phone @ 859-227-7099). They will discuss your needs and suggest possible solutions, including building one for your specific situation.
For lighter and less expensive lifts that might be more compatible with large trucks and equipment, check out: the Coach Lift, made by SS Products, of Tempe, AZ (by phone @ 888-224-1425 or on the Web @ www.coachlift.com and click on "Farm Handicapped Lift") and the Glide 'n Go Power Seat Lift, made by Access Unlimited, of Binghamton, NY (by phone @ 800-849-2143 or on the Web @ www.accessunlimited.com and click on "Other Vehicle Products).
The Breaking New Ground Resource Center at Purdue University also has printed materials that address this topic. They include the following four fact sheets found in The Toolbox (a CD-ROM and on-line catalog of modified equipment and assistive devices for farming with a disability): "Commercially Manufactured Lifts," "Independent Mounted Lifts," "Equipment-Mounted Chair Lifts," and "Equipment-Mounted Platform Lifts." Your county Extension office and local high school agricultural education teacher should have a copy of The Toolbox CD to review (or perhaps borrow); also you can access it (in pdf format) on BNG's Web site—www.bngtoolbox.info.
There's a possibility that your state's rehabilitation services or vocational rehabilitation program will purchase, or at least help you purchase the man-lift(s). To find out, contact them either by phone (look in the yellow pages under "Government, State" or in a separate section ahead of the white pages that lists state government offices) or via the Web (go to www.agrability.org and click on "Resources" then "Vocational Rehabilitation contact information."
* Note: When modifying truck for use by someone with a mobility impairment, extra precautions are needed to reduce the risk of injury.