GVWR (and payload, which is calculated from it) is often just a legal number for registration, licensing, insurance, etc. purposes. The GVWR of my truck (see signature) is 14,000 lbs which makes it a Class 3 truck. If it were 14,001 or higher, it would be a Class 4 truck. The combined GAWRs, on the other hand, are 6,000 + 9,750 = 15,750 lbs.
The rear axle GAWR is a hard limit that I wouldn't exceed, especially if it's based on tire ratings. The trailer length is irrelevant - my "38 foot" (actually 40') trailer has a 19,000 lb GVWR. The same model Elite Suites has a 21,000 lb GVWR. There are similar length 5th wheels on the market that may only have a 15,000 or 16,000 lb GVWR. Loaded pin weight of the trailer and the truck's rear axle GAWR will be what determines your maximum trailer weight with a SRW truck.
Rusty
