Hmmm. The purple worm is the same length and 40,000 lb., but I think it may be a bit smaller in cross section. On the other hand, the many-humped has air sacs, so maybe it's less dense. Want to go with 40,000-50,000 lb?
Free-swimming aquatic animals normally have the same density as water, otherwise they waste energy to prevent themselves sinking down (if heavier) or floating up (if lighter).
A Purple Worm averages 5 feet in diameter and 80 feet in length according to the SRD. If it's a simple cylinder, that'd work out as 1570 cubic feet, or roughly 25 pounds per cubic feet at its listed weight of 40,000 pounds. That seems too low density. If the worm tapers a bit at one end or another it'll have less volume and therefore have to a higher density. For example, if only the first 20 feet (the "business end") was a uniform cylinder and the remaining 60 feet (the "tail end") tapered evenly to a point and so has the volume of a cone, its total volume would be halved, making it 50 pounds a cubic feet.
That was getting a bit beside the point though - which is the Many-Humped ought to have a respectable weight.
I was thinking around 40 tons would be acceptable, which'd make it around 6 feet in diameter (assuming it's as dense as water and its 80 foot length approximates to 40% cylinder and 60% cone),
For comparison, that works out as two-fifth the weight of our Merhorse conversion if it were scaled up to the same length. It also means its body proportions are twice as thick as our
Titanoboa "giant boa constrictor" conversion (which was 80 feet long and 10 tons).
A many-humped sea serpent is a type of primitive whale. It has a single pair of flippers and a small fin near its head and a double-lobed tail. Along its back, stretching the length of the body, is a regular set of large humps, which are believed to be hydrostatic organs that provide stability as well as a reserve of air for long dives.
That rather mixed up the "description" and "background" portions of the fluff.
I'm not feeling desperately creative at the moment. (Partly because one of my blinkin' cats woke me up three times last night.)
If you're lucky I'll post a draft for some fluff later on.