We charge the same for in shop or on the road labor hours, and most places I have worked over the years do the same, we base this as much upon what the techs are paid (equally depending on skill sets and experience), as any other factor, and we would not want the road techs to not be as willing to sell shop jobs because they don't bring as much per hour. We do use the shop as where most of our apprentices do their learning with more supervision.
IMHO, the cost for facilities should be shared equally between the road and shop, since a lot of what happens in the facility are support functions for the road service (management, parts, and dispatch, etc..).
as far as; "Doing all repairs on-site offers many economic benefits to both parties" I would say 'false" mainly because of the "ALL" repairs. I don't think there is any repair that -requires- my shop, but I do firmly believe the best use of assets would dictate that some/most longer jobs are best done where the machine can sit out of the way while subsystems are being repaired (I almost never pull an engine out on site, if the transportation of the entire forklift would not take any longer than transporting the engine).