I have a Whirlpool model LER8648LW0 that is about 8 years old. It gets a lot of use. It used to dry well, but now takes almost twice as long to dry an average load. Towels take about 1 1/2 hours on high heat. It gets hot, but doesn't seem to be as hot as it used to on high. Is it a heater element or could it be something else?

Sounds like your problem may be with the heating coil.
If you plan to try to repair the problem yourself, then the first thing you want to do is unplug the dryer from the electric.
Remove the back of the machine and try to locate the air tube going from the blower to the drum. Normally inside the air tube you will find the heating coil and the thermostats/thermistor.
Once you have located the heating coil, remove the two wires from it and check the coil with an ohm meter across the two terminals of the coil.
You should read continuity across the terminals, if not you will need to replace the coil as it is faulty.
If the coil is ok, then you want to check the thermistor/thermostats. Remove the wires and tape them together one at a time with electrical tape and turn on the machine and see if it heats up ok...if it does then replace the thermostat/thermistor that is bad.
DO NOT LEAVE THEM TAPED TOGETHER. BECAUSE THIS BY-PASSES THE SAFETY OF THE MACHINE.
Good Luck,
Hope this helps