2 x car batteries.
I have two old batteries from two previous cars. The batteries were not that good at holding their charge while strapped into the cars but seem perfectly capable of charging various gadgets (mainly phones) via a 3-way USB adapter, which has 3 x twin USB adapters so thereby giving a total of 6 charging points. And even that's never enough!
I keep them charged & conditioned for a while before leaving for LM and that way they provide most, if not all, of our on-site charging needs for 4 or 5 days.
That reminds me, I must go and start the charging & conditioning...