The long string of nouns and adjectives make the phrases sound stiff and a bit problematic to parse. I also suspect that readers would assume someone campsite would be onsite (also spelt as one word).
The campsite stewards are first-aid trained.
The campsite stewards are first aid trained (without hyphens)
The on-site stewards are trained in first aid.
If the OP prefers a noun phrase:
first-aid trained campsite stewards
onsite first-aid-trained campsite stewards
This source uses the unhyphenated “first aid trained” several instances even when it is an attributive noun phrase.
First aid trained individuals can triage injuries, control the scene to keep others safe, and call for emergency services when needed. Not all first aid trained employees will feel comfortable performing CPR, using an automated external defibrillator (AED), or splinting a broken bone, however.
Source: J. J. Keller®COMPLIANCE NETWORK
@EdwinAshworth I think the solution with the en-dash is probably clearest, thank you.
– Rosie Mar 19 '24 at 14:30