As I do not have a copy of a current service/warranty book cannot be specific, but as the warranty is effectively a consumer contract, it would fall under the old SoGA or the current CRA regulations for the standard and fairness of its customer requirements and effect. This means it has to be reasonable and fair.
With that in mind, it could be unfair if it stipulated the customer had to carry out a particular action (such as a service) when usual prevailing circumstances under the general control of the contract supplier make it difficult to do so (such as dealerships being unable to book service slots).
It is clear the caravan manufacturer is aware of the difficulties of booking any of the intermediate services on the due date, because they do give quite a generous +/- 90 days window, it would have been reasonable to assume the same latitude would apply to the final service, But because they place a specified clause in the T&C's which sets out a different criteria for its date, that clause is important and should be considered on its own. The question is:- Is it reasonable to place such a tight deadline for a customer action, when service slots are known to be difficult to find? Alternatively what would a reasonable leeway be for this final service? Without knowing the manufacturers reason for making the such a specific requirement, what might be a reasonable variance is unknown.