Maybe the best way to think of it is that the quoted length is for the RV pad, i.e. the portion of the site intended to hold the RV itself. There may be more room on the site, but maybe not. The park operators know that most campers will have at least one vehicle besides the "camping unit" and some will stipulate that the site accommodates one (or more) vehicles. Others, however don't say and may have applied other value judgements such as what John alluded to.
There is no standard method of stating it, nor even of measuring how long it is. One common variant is the rear overhang of the RV (behind the axles). Many sites will have room to let the rear of the extend beyond the actual site "pad", but this extra space is almost never counted in the measurement. For that reason, it may well be possible to put a 35+ foot RV on a 30 foot site.