I don't think it is a legal issue, unless you are somewhere that GCWR is used for tax/license purposes.
At 14,500 GVWR, the chassis was a P-series (the G-30 went only to 12,300). In later years, GM made a practice of rating GCWR = GVWR + 3500 for the P-series chassis in RV applications.
I have not found any documents for the 1980s. GCWRs for Class 3 GM cab-chassis with the 454 were typically in the 16,000-19,000 range, could go higher with gearing and manual transmission choices. The RVs got automatics only (TH-400).
I don't think many manufacturers did tow ratings for gas motorhomes in the 1980s. Towing a dinghy was something that developed a little later as RV lifestyles evolved from vacation travel toward full time living. As that happened, RV manufacturers started using chassis with better power to weight and overall size grew as well.
