Basically a 5th wheel is like a house on wheels. The same items reliable in your house will be reliable in your 5th Wheel. Majority of RV'ers, if buying new, will have 5-10 years of relatively trouble free travel as long as preventative maintenance is performed. After 5 years you'll have small things go wrong...plumbing leaks, microwave oven failure, electrical issues, trim issues...etc. After 10 years then the big stuff begins to happen; roof leaks, refrigerator or air conditioner fail, paint/graphics problems as well as suspension and brake issues.
In the mean time you've had 10 years of fun travelling around the country seeing the sights. It's well worth every penny...