Having just sold an Alping, a top of the line 5th wheel recently I can tell you this: check it over thoroughly, have the owner run all the systems to your satisfaction and show you how to operate them. Inspect all outsides of the rv including the roof and the undercarriage. Look for signs of recent work on the rv, leaks from the water, holding tanks, hydraulics, etc. Check the tires for dry rot and tread. Other than these things it should be straight forward, enjoy your purchase, enjoy your rv. We enjoyed over 20 years of rving.
'05 Keystone Challenger TPK
'01 Chev 2500HD Duramax/Allison
Corgi "Pippin" & cat "Sundance":)