Use it first, see what shows up...there shouldn't be anything "major"
in a 1998. If there is, you will notice it after a half dozen nights out or the routine "snooping around" all newbies do.
The only reason I wouldn't
use it first is if you don't feel comfortable hooking up the gas/elec/water without some hand-holding. In that case, I'd say go to a rally or gathering of forum members to gain some confidence first. Save your $$$, use up the eager free advice first...then go to a dealer or RV Tech if you find something amiss.
BTW, nice of the PO to arrange a "leak check" during your inspection! Enjoy your new trailer!!!