Tires: The tires must be replaced. Trailer tires should be replaced before 5 years of years of service regardless of tread depth or tire appearance.
Underbelly - I do not know if his is enclosed or not but the later models are enclosed.
Appliances: This is really hard to tell. I don't think there is a specific range because it has a lot to do with how is was used and maintained. Especially the fridge - for instance, running the fridge is an unlevel position will causes it to fail prematurely.
The most important thing to check on any camper - new or used - will be the seals on the roof and sides. Check everything.
Climb up onto the roof and inspect every inch of seam, gasket, and any other placed that the roof material has been cut or holes drilled. Check closely around A/C, vents, antennas, sewer vent, and side seams. Make sure the caulk looks good and that its not cracked a dried.
Inspect for siding issues such as delamination, color variation, dents or irregularities. Also inspect the inside walls and ceiling for any discoloration or bubbling that could indicate water intrusion. Feel around for soft spots.
With the slide, the best you can do is operate it a few times on both shore power (AC) and battery (DC). Listen for signs of unusual rubbing or if its struggling (It is normal for some slides to more harder for the first few inches from their fully retracted position or fully extended position. That is somewhat normal, but the rest ought to be smooth with no shuddering). Check the floor to make sure it hasn't rubbed a hole into the carpet or linoleum that could indicate it needs to be adjusted. Check the seals and note any gaps on the top, bottom, or sides where water can enter.
I would also ask how old the battery is. That might need to be replaced.
That's just the basics and you would do best to get a PDI checklist. Here are the results
from a quick search for those.
Good luck and have fun today. Hope this post isn't too late.