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There's nothing new under the sun and - likely - someone else already figured this out, but here it is anyway:

When playing the 8th partial second valve pitch,
"high"
A - B-flat tuba
B - C tuba
D - E-flat tuba
E - F tuba
...and trilling down a semitone,
Hold down the second valve, and wiggle the fourth valve. This is quite a bit less awkward than alternating between first and second valves.

Have fun.

Here's another one - though I'm not sure that it's needed as the standard valve combinations usually works pretty well.

The standard fingering for this trill is the same as the previous: "quickly alternating between 2 and 1", but - if that's difficult, or doesn't work smoothing...and you'll need a long whole tone 5th valve for this

5th partial 2nd valve pitch: (trilling down a semitone from these pitches)

D-flat (B-flat tuba)

E-flat (C tuba)

F-sharp (E-flat tuba)

A-flat (F tuba)

repeating:
...If trilling DOWN a semitone from these pitches - and quickly alternating between 2 and 1 doesn't work well for you and you have a long whole tone 5th valve on board...

try the valve combination 5-1 for the primary pitch (holding down those two valves) and wiggling the 2nd valve - for a downward semitone trill.