I use a software program called Fleet Maintenance Pro. I started using it when I had to keep service records for a fleet of cars and trucks. Thankfully I no longer have to do that. However I still use it for all our personal cars, trucks, tractors, Motorhome, etc, etc. I currently track all the maintenance intervals and costs for 57 pieces of equipment.
It's a fairly sophisticated program in that it can track not only the service end of things, but also the fuel cost, insurance, licenses, permits, and an inventory of over a thousand parts. You can track everything by time or miles. It even has a feature that gives alerts when maintenance intervals are near.
They also make 2 personal versions that can track 5 or 10 vehicles. The personal versions are extremely reasonably priced @ $34.95 for the 5 vehicle version and $49.95 for the 10 vehicle version.
Here's a link to their site: Fleet Maintenance Pro, Maintenance Pro, and Auto Maintenance Pro - pricing and ordering
I've been using their software for over a dozen years. It's easy to setup and use. The nice part is all the information is at your finger tips.
I have no association with this company other than the fact that I purchased and regularly use their software.