No.....we have a Bailey and I rang them regarding this and was told that it did NOT have to be serviced by a Bailey dealer but MUST be serviced by a NCC (National Caravan Council) registered dealer, which is what we did....and the dealers who did the service found severe water ingress which Bailey paid for under the 6 year warranty....
If your still in doubt, ring customer services at Bailey...very helpful !
NCC have a website and list their members on it.
Dont know where you live but I used one in Brighouse (West Yorkshire)....First Class