Why not purchase an entire Allen wrench set, not that expensive. If that won't work, I would go to the Valterra site and download the install manual. Maybe they indicate the size Allen wrench needed.
Sorry I can help with size.
Steve Rinker
Rochester, NY
2013 Keystone Sydney 340FBH 5th Wheel, 12,280 lbs loaded (scale)
2015.5 GMC Sierra Denali 3500, SRW, Duramax, CC, Payload 3,700 (sticker- not scaled yet)
Take my posts for what they are, opinions based on my own experiences.