Here is a link to your parts breakdown
Whirlpool IEX3000RQ0
Your cycling thermostat is Item 24 in the Bulkhead section. Click on it's part number (blue print) to get more info about the part.
The pantyhose works great as it will let you check temp and air flow.
Leave enough room for it to blow out like a windsock.
Also if it runs OK on the pantyhose you know it is a vent problem.
Slow dry can have 3 causes
1. Clothes too damp - I assume you have checked this but sometimes what appears to be a dryer problem is actually a washer problem.
2. Temperature - With 30 years experience I would trust the service person that the temp is OK. The cycling thermostat can go bad in a number of ways:
A: trip temperatures change - very rare but does happen - yours looks like it is adjustable. Move the adjustment from one end to the other, then reset it just slightly higher than it's original setting.
B. contacts shorted - unit will then run on the thermal limit thermostat - exhaust very hot and a large temperature drop before it restarts the heater
C. Contacts open - you would not get heat
3. Air Flow - Air flow should be good and strong. Things that can mess it up are a blower wheel clogged up with lint, broken blower wheel fins or the blower wheel is loose on the motor shaft.
Also if your felt seals at the front and back of the drum are leaking you will not get the air going through the drum to dry the clothes. Take a thin piece of cardboard (cereal box top) and run it around the drum front and back to make sure the seals are doing their job.
Check the air exhaust grill, in side the drum area at the back of the unit, make sure all the openings are clear. They can clog up especially if you are a little heavy with the fabric softener. This also applies to the lint filter.