A holding tank stores what goes down the heads until it's pumped out at a station or discharged where that's allowed. Around it are hoses to the toilet, a deck pumpout fitting, often a seacock and pump for discharge at sea, and a vent.
The usual complaint is smell. Practical Sailor's finding is that it mostly comes from hoses and fittings rather than the tank.
What a surveyor checks
Look at the tank, its seams and fittings for weeping, and the bilge below for staining. Note the material: Peggie Hall, in Power & Motoryacht, says metal tanks typically start leaking at a seam or fitting within two to five years, and recommends rotomoulded polyethylene. Check the hose ends and clamps, and do Practical Sailor's damp rag test on the hose: wipe it, sniff the rag, and permeated hose gives itself away. Check the vent is clear.
In the report
"Holding tank, stainless, weeping at the outlet fitting with staining in the bilge below. Sanitation hose permeated, fails the rag test. Replace the tank with a polyethylene one and renew the hoses." See holding tank leak.