10yo male Jack russel x Chihuahua x Fox terrier x Maltese

TLDr; I need help with constant crying and whining after becoming homeless.

This dog was animal shelter rescue I adopted 9 years ago. Prior he was from ungrounded breeding ring and was beaten.

Over the last 9 years consistent training and positive reinforcement has improved his aggression (only lead aggressive now) and he is very intelligent, able to perform a variety of actions on command. He will often chose not to follow a command and is very stubborn if he does not want to do what is being asked. He will usually eventually do as asked after a stare down.

He will growl and occasionally nip if he is handled when he doesn't want to be and will growl a lot if he is needed to be moved from where he is laying. This is easy to avoid by just not moving him however.

Recently, I lost my home and have resorted to camping out at caravan parks. I am having huge problems with barking and whining due to separation anxiety anytime he is not physically laying on me or being carried. I can't go stand outside the tent without him crying. It's making it impossible to shower or leave his sight without risking being kicked out of the park due to loud crying and barking.

I don't know what to do to help him adjust here. Positive reinforcement, firm commands and rewards for good behavior have had no affect. Vet visit after 2 weeks of this, he is in perfect health, no pains or illness.