When the Citroen BX was released in the early 1980's the recommended change mileage was 100,000 km I believe - a 'high' mileage - or kilometerage if you prefer. It was French so it was all Km. A Solicitor bought one and had it serviced at the recommended intervals at a franchised Citroen dealer. At 60,000km the belt broke and the car needed a new engine. Unsurprisingly Citroen said "Non, sur votre bicyclette" so Monsieur Solicitor took them to court saying he had done all the maintenance specified, at the manufacturer's agent. He won and Citroen was made to change the engine free of charge. They then promptly reduced the maintenance specification for the cam belt change to 30,000 km (I think my numbers are right but if you know better please fill me in).
When I got my current 2009 Transit based motorhome two years ago it had about 30,000 miles on the clock and two auxiliary belts. One had been renewed at some time but the other had not. That belt had hairline cracks all across it, and so it was time for it to be changed. I tried and failed twice to change it because of one particularly difficult bolt I couldn't undo. Then I thought that if I take it to a garage and pay them an hour labour plus the belt, they would do it. So it must be do-able and I worked out that if I removed the air con pump I could get at the bolt and I succeeded in changing it.
This engine, the Ford 2.2 diesel Euro4 I am assured has a valve timing chain, so I have no intention of looking! However if you have a rubber belt, you can usually remove a few bolts securing the black plastic housing around the belt somewhere and inspect it. If it has cracks then change it.
It depends if you can do it yourself in which case the £80 or whatever it is cost for a whole kit including idler wheel and water pump is well worth the effort of doing. If you have to pay a garage several hundred pounds, phone round a few. I had one vehicle that needed it and there was a huge difference between garages.
Think of it as the same as changing tyres when the walls are cracked. You can spend lots of hundreds of pounds changing them all or you can risk a blow out on the motorway.
One alternative which a lot of people adopt is: "My tyres are cracked and my cam belt has been there for 60,000 miles and the engine
battery doesn't turn over very fast - £900" and trade it for a newer model with everything ticketty boo.
Which in turn goes a long way to justifying buying a used motorhome from a dealer - and do see if you can find reviews of the dealer before buying . . .
As a last thought, the early Mercedes car engines and presumably commercials, had duplex timing chains. The owner of CAK tanks told me he had a diesel van, one of those with a sticky-out nose, and at 500,000 miles he thought it might need an overhaul. He removed and dismantled the engine, cleaned everything and reassembled it with new seals. At 1,000,000 miles he sold it. A couple of years later he met the buyer who told him that the van was still running, at about 3 million miles. Presumably the driver was held sedated in some secure accommodation somewhere.