I work at an RV dealer and to a certain degree, this is true. Often times, Peter is robbed to pay Paul on a used RV delivery. If the battery
is bad on a sold unit, one (Or in the case of motorhomes 3 to 4) batteries will get pulled from a unit in stock to get the customer out the door. The replacement battery for Peter may or may not be new.
In the case of tires
, A/Cs or any other part that requires a ton of labor to "replace".. this is not true. It costs more to pay the RV tech to do the swap than it does in perceived savings on a new part.
Most RV Dealers do not have the facilities to do tires either. The rig gets driven to a shop.