This type of agreement has been around for a long time. The key to your success in offering this type of agreement is how you calculate the monthly charge which needs to take into consideration the type of truck (Gas, LPG, Diesel, Electric etc) as well as the quality and durability of the truck, cost of parts, cost of labor, the severity of the application (clean warehouse, good or bad floors, used inside or outside or both etc), are the drivers trained well, do they have assigned trucks or not, distance from your service facility, if it is an electric truck do they have an adequate charging area and practice good battery usage (watering them when needed, not over discharging them etc.). What are the manufacturers suggested service intervals? How long and what is covered during the warranty period? What is covered and what is not (as was mentioned in a earlier post, usually abuse or improper use of the truck that results in damage is not covered. In many cases wheels and tires are not covered either. If you represent a major lift truck OEM, they should have some information that can help you in your calculations. If you do not represent an OEM you will have to approximate how many labor hours and parts dollars will be needed for breakdown repairs, PM's, travel time etc. The other cost is inflation. If you calculate your costs in today's dollars how much do you anticipate those costs increasing over the subsequent years of the contract? Inflation has been relatively low in recent years however that may not be the case going forward. A guesstimate would be 3 to 3.5% per year.
The other thing to make sure you do is to reserve a percentage of the money that you will collect in the early years of the contract to cover the higher maintenance costs that will occur in the later years. The more of these types of contracts that you sell will help you spread your risk out providing you have done the calculations correctly. There will always be some of these agreement that are winners and some that will be losers. The idea is to have more winners than losers!
As to contracts, if you represent a major OEM, they probably already have standard contract that you can use. If not, the best thing is to hire a lawyer to make sure you wind up with an agreement that will protect you and be fair to your customer.
At the end of the day full maintenance contracts can be (and usually are) profitable if they are managed properly. They also help prevent you competitors from taking the business away from you during the contracts duration.
I wish you the best with your customer!