All right, so lets get a little technical,
There are three issues that are related but their relationship is frequently misunderstood. Caravan Service, Warranty Repairs and Manufactures Guarantee.
As with any mechanical device, its continued good service will depend on timely maintenance. As it is not within the scope of most end users to plan a maintenance schedule based on changes in direct or indirect performance indicators, the Caravan Manufactures simplify the process by suggesting that a schedule based on a combination of time and distance is adopted. There is no legal obligation to follow this prescription, however for a road going vehicle it should be maintained in a road-worthy condition.
It should also be noted that a caravan manufacture couldn't force an end user to follow the prescribed service schedule, or to dictate who should carry out the service. That is entirely at the discretion of the owner, HOWEVER, if you wish to take advantage of the Manufactures Guarantee, you must abide by the terms laid down by the manufacture.
There is no legal requirement for a manufacture to offer a guarantee to the end user. By doing so, the manufacture is offering a 'gift', but they make it a conditional contract. They may withdraw the gift if the conditions are not maintained. Consequently if the conditions call for the caravan to be serviced at particular stages in it life or travels, and that it must be carried out by a specified business, if you fail to comply, you have broken the contract and the manufacture is perfectly within their rights to withdraw their guarantee.
There is always an often-overlooked phrase in every manufactures guarantee wording. Something to the effect of 'This guarantee does not affect your statutory rights' This is a vital and important phrase because it shows that the mighty manufactures are not all powerful and they have to acknowledge it, but it doesn't directly affect them:
Your statutory rights are detailed in the Sale Of Goods Acts, (see http://www.dti.gov.uk/ccp/topics1/facts/salegoodsact.htm). In summary the act defines that the seller is responsible for the goods being 'as described' and 'fit for purpose' If your purchase is not compliant with either of these issues, your redress is to the seller. You will also note that that your rights under this act are not limited to a specific time period following sale, so it is not a one-year warranty, but each case would be assessed on its own merits. It is also important to understand that as the user you have to treat the product reasonably, as abuse would reduce your rights under the legislation. If there is dispute, then the case can be settled in court.
You will also note that your redress in the event of an unreasonable product failure is to the seller not direct to the manufacturer. It may surprise some people to learn that if you use an HP arrangement to finance your caravan, then technically the finance house is the 'seller' and not the supplying dealer. If in doubt, always check with a solicitor, Citizens Advice Bureau or Trading Standards.
Even armed with all this information, if you want to maintain the Manufactures Guarantee, you must comply with their conditions, even if it means travelling to a particular service agent.
If travelling to maintain the Manufactures Guarantee is an issue, then you should assess its impact before making the purchase. If your lordship is determined to have a particular type of van only available from a remote dealer, then that is choice you must make and be prepared to pay the price of that distance.
When dealing with service agents, it always good practice to confirm they have the parts available (especially special order items) before you set out.
In an earlier reply I hinted that there are challenges being made with regard to the manufactures insistence on returning to a particular dealer for service work. There have been similar issues with regard to car servicing, but the outcome of the challenges is complex, and sadly it does not set clear guidance. Further cases will hopefully add clarity and strengthen the consumer's arm.
Despite this it may be worth writing to the manufacture to se if the terms of the Guarantee can be amended, if enough people show interest, the manufacture may decide to amend their conditions for good customer relations.