The trailer gone probably because server's vehicle got respawned by a staff. About the shooting range bug I think it's a SAMP bug.
Issue regarding trailer respawning fixed. Admin command to respawn unoccupied vehicles will now omit mission vehicles that aren't being driven.
Issue regarding shooting range targets is a SA-MP/GTA issue and unfixable.
Thank you for your reports, however in the future please post each issue in an individual topic, as I have fixed one issue but not the other, and now cannot properly categorize this topic in fixed/unfixed.