How much maintenance for car that is rarely driven? What REALLY needs doing?

What is your question?

I drive my car no at the most 50 miles a month, and have only driven it 5,000 miles in the past 5 years (the car has 93k miles total). Because I hardly ever drive it, I rarely have any maintenance done until a problem shows itself. I very rarely change the oil but use synthetic when I do. Any time I have had anything done I ask them to look at all systems, but the only real maintenance I have had done is have had the front brakes replaced. I did not complete the 90k mileage service because the list of things they do is all "Check", and it costs $200-300 and it seems they don't much of anything (also every mechanic seems to try and charge me for stuff I don't need). I only get about 14 miles to the gallon, but all my driving is short, city trips with a lot of rapid acceleration. I would be concerned to take the car on any long trip, but I would like to feel confident that if I want to drive a long car trip that I won't have a serious problem. So, I have two questions: 1) Should I have the 90k service completed? 2) What maintenance's are the most important that I should consider? 3) Is the "penny test" for the tire tread legitimate? Thanks so much for any help you can give me. I am disabled, which is why I don't drive much and don't have a lot of money for car maintenance.