IF tires are over 6 years then it is a good time to change them all, no matter the wear. If less than 6 then you may be alright. Cracks on the outside are different than issues with the tire's interior, but cracks can indicate over-all tire condition, inside and out. Nobody can say for sure about your tires. Old tires will blow out. New tires should not blow up.
Worst case is the tire blows out and the flaps of steel and rubber do damage to your fender or the side, or under your trailer. Best case is the tire does not blow out, but just goes flat.
I had a blow out on a used trailer I was just trying to get home so i could put on new tires. I towed it for miles before I even knew the tire had gone since it was a dual axle trailer. The tire flap did damage under the fender well. These tires were very very old though.
People replace tires after 6 years because the risk of tire damage to the camper is not worth the cost of fixing the camper, not because of cracks.