RV folks started the 80 % recommendation years ago when hp and torque numbers were low. Diesel had maybe 180 hp and 460 torque.
The big block 460/454 Ford and GM gazzers had maybe 230-240 hp and 370-390 torque ....hence the 80 percent recommendation worked better.
It's true engines were weaker back in the day, but the tow ratings were lower as well, so there is still a proportion of some sort. Further, more than just engine torque & hp goes into a tow rating. That said, the rule of thumb percentage is indeed somewhat obsolete, for reasons I will explain below.
The percentage originated as a shortcut that recognizes that the tow rating is based on a tow vehicle that is empty except for a 154 lb driver and full fuel. Hardly anybody tows that way, and any extra passengers and gear (including the hitch) decreases the tow capacity by the same amount. If the tow vehicle had a 8,000 lb tow rating but there was an extra 500 lbs in it, the 8,000 is reduced to 7,500. That's about 6% reduction. We used 10% as an easy-to-use percentage and then doubled that to 20% IF towing in the mountains, because engine horsepower decreases with altitude even though the horsepower demand for towing increases. So 90% worked on the flatlands and 80% was better in the mountains.
Another point: The 80% & 90% factors were developed when there were mostly travel trailers and hitch weights were not such as huge factor as they are with a 5W trailer. The 80% rule never had much value for 5W towing because it ignores pin weights and payload, and the truck ofteny ran out of payload before reaching even 80% of max tow rating.
Two things have changed the percentage parameter. First, the SAE 2807 towing standard has been adopted and that requires that the tow rating allow for two persons and the hitch as well as fuel. That puts the tow rating much closer to real life driving and thus reduces the need to compensate somewhat. Second, the substantial increase in tow ratings due to higher power, improved transmissions and better suspensions makes the load in the typical tow vehicle a smaller percentage of the total load. That suggests a smaller percentage would be more accurate, especially in the more powerful tow vehicles. An extra 500 lbs in an F350 with a 20,000 lb tow rating is only 2.5%. The same 500 though, is a whopping 10% if the tow rating is only 5000 lbs (as it would be in a mid-size SUV, for example).
Any rule of thumb must have two attributes to become popular & useful: (1) It must be simple to remember and use, and (2) it must be applicable across a broad range of scenarios without getting anybody into serious trouble. Accuracy takes a second seat to both #1 & #2 and the rules typically err on the side of caution.
