in my end of the world we call that a TM&R contract, generally setup for 5yrs or 10,000 hrs whichever comes first.
It generally covers all maintenance and repairs except for customer abuse, warranty or tires. There are certain high wear items it will not cover like brake shoes, batteries etc. unless it is a defect causing the failure.
As far as a form contract? i'm pretty sure these are tailored to each manufacturer criteria on a case per case basis but from what I've seen they are fairly similar across the board.
I'm not privy on how these contracts are calculated but i'm pretty sure the cost per month, the estimated maintenance and repairs are pre-calculated into what the customer pays per month and probably at a discounted flat rate.

Now if you are not a dealer that has the capitol to cover this or have factory backing i would not enter into this agreement as an independent service provider because in some cases i have seen situations where the job application was very hard on the trucks and there were alot of failures (not abuse) that created alot of repairs on the trucks and the cost was extensive over time. A case such as this would be financially devastating to you in the long run.