Back when I had campervans I found Safeguard were good, and they were happy to include business use without additional cost when I requested it.

Mind you I wasn't, and am not, a musician, so my best suggestion is that you ask them. Insurers are very wary of musicians, also actors, licensees and one or two other groups. You can expect a premium loading from most insurers who are ready to quote, and will likely find that many will simply decline to quote.