Any Approved Workshop engineer can service vans within warranty and retain the warranty.
However, that applies ONLY to servicing, if any faults are found with the structure of the van , it is the SUPPLYING dealers responsibility to repair, and if you have had a mobile engineer do the servicing, don't expect your supplying dealer to be very happy to only be asked to do warranty work.
Faulty appliances can usually be repaired by a mobile engineer as the warranty is with the appliance manufacturer and most of the AWS engineers are authorised to carry out warranty repairs on faulty appliances.
For an engineer near you look up Approved Workshops and select your area on the Find An Engineer tab:
http://www.approvedworkshops.co.uk/search/
Footnote:
Dustydog has posted information from the Elddis T&C's, but Elddis are supporters of the AWS scheme and as such Approved Workshop Mobile Engineers CAN carry out servicing and retain the warranty.