Going camping

It is common in the US to see in the interstate highways monster-large recreational vehicles. In many cases they are towing a car or have a motorcycle hung on the rear so when people camp they still have a means of moving around. In the 1920's however, one could encounter the opposite spectacle: an RV being towed by a motorcycle! When I see pictures like this I wonder if the hp was modified in its definition since the 1920's. In the text accompanying this picture they claim the 8hp Royal Enfield v-twin motorcycle is plenty enough to tow a sidecar AND a 400 pound camping trailer. Granted, 400 pound is hyper-light by today's US standards (most "light" camping trailers nowadays weigh 1500 pounds or more). But still...