Time to replace those tyres I'm afraid!!Even if the tyres have little mileage and good tread left, after 5 years the manufacturer will not entertain any claim for damages.
The problem is that the rubber compound starts to deteriorate with age and can in an extreme case even separate causing the disastrous blowouts that we so often hear about.
I am not sure what tyre the '92 Palma was fitted with but my guess is at 12 years old it was probably an underperforming sedan tyre not suited to the load carrying requirements of the caravan axle.
If you cannot read the tyre size, take one off and ask your tyre dealer to source you the corresponding 6ply steelbelt commercial (light truck or bakkie) tyre that is available.