With regards to the Jayco Dealer in Darwin, things may have changed in the last few weeks but until then this Dealer wasn't a Service Agent for anything, just a mixed car and caravan sales yard. I do believe though that a few weeks ago a representative of Jayco was up here and discussions were to take place with regard to warranty work. The Jayco website does have the Dealer as a Service Agent so it may have changed.
Alternatively if anyone wants a mobile service in general then you should contact Paul Crooke, Mobile Caravan Repairs on 0438 120 571. AFAIK he is the only mobile service up here and as of 7 weeks ago he was also the person that Jayco HQ referred me to. He is quite busy though but I found the price he charged to be quite reasonable for a bearing repack and brake adjustment. As a last resort there is the Coromal Dealer at Pinelands (a few Km's South of the Jayco Dealer) who did offer to do the job when I couldn't contact Paul for a while. I would have had to take it to them though.
BTW, we bought our van in Adelaide rather than locally and Paul would be my choice for service.