Airline employees, both at the airport and on the plane, are not tipped. If you employ the services of a porter at an airport then you may wish to consider tipping. However, hardly any airports provide porters these days - nearly everyone carries their own luggage. Employees of car rental companies are not tipped, except possibly the driver of the courtesy bus which takes you between the airport terminal and the car rental company's depot. I have never personally seen anyone tip a car rental courtesy bus driver, but some people have told me that they do tip in this situation.
Most gas stations have both self service and full service pumps.
The price is slightly higher at full service pumps, so you should never tip. Even if you stay at the same motel for several days, you are unlikely to meet any employee of the motel except when you check in, and perhaps when you check out. In many motels you pay when you check in and you leave the key in your room or drop it in a box near the office when you leave. Therefore tipping at motels is almost totally unheard of. In a few motels the people who tidy the rooms leave an envelope for tips, but this is very rare.