AGM batteries self discharge very slowly, though most modern vehicles have small but finite parasitic loads such as the central locking remote control receiver. Most motorcycles are genuinely "off" when turned off and anyway they still have permanent magnet alternators and carburettors. Small pull start gen sets aren't that uncommon. Lots of motorhomes and yachts have solar panels, heck lots of buildings have solar panels these days. Finding "seed power" to start your way up to bigger things shouldn't be difficult.