The 1/8 drill bit is sufficient for whacking off the head
of most rivets, which is how they're removed. If properly done, the rest of the rivet just slides out and leaves behind a hole of the originally appropriate diameter....and evidently Scamp used a variety in that department. But that's the only
trailer-wide common use of the 1/8 bit!
Scamp's rivet page
indicates that they're differentiated not just by length but by shaft diameter/size. The diameter of the rivet's hole combined with the depth of the surface it has to go through determines the size of the rivet to be used.
Hole size/rivet diameter are important mostly for load bearing- the bigger the rivet the heavier the load.
Length is important since there are limits within which the rivet will effectively operate, and one that's too long will NOT tighten up properly.
Since Scamp's webpage offers rivets in eight sizes, it's fair to assume that's how many you'll encounter on your trailer. You might cut down on necessity for many different shaft diameters by committing to replacement of all with the biggest diameter (heaviest load-bearing), provided
that diameter is available in the lengths you need.
It's a simple enough matter to drill smaller holes out to the bigger diameter...
This of course assumes that the cosmetic caps are of a uniform
size, and not conspicuously different for different places.
Though length is pretty much cast in stone, you can fudge it a bit with a longer rivet by use of aluminum washers
on the blind side. Not the best practice, but it works, as I can attest!