Bill, I do not know specifically about Phantom, except some adverse comments by people who have had it fitted, only to find that it had stopped working without their knowledge!!
Having seen the inside of one unit, al it is , is a mobile phone with a larger battery taped to it.
I would suggest that you may wish to consider a new tracker on the market, at present being fitted to the top range Swift caravans and also Airstream.
It is called HAL Locate, see here http://www.locate-sales.com/products.php
It is th eonly truly dedicated caravan tracker system and has Thatcham Approval, again the only one available.
I have seen this unit in operation and it is impressive.
It will even text you when your leisure battery needs charging!!
The cost is not cheap, but depending on how long a contract you take, the cheaper it works out.
The unit itself is about the size of a credit card, so very discreet, and uses a dedicated GPS aerial to give precise latitude and logitude location and is monitored by Euro Watch, the largest monitoring outfit in th ewhole of Europe, and covers over 40 countries, unlike Phantom which is primarily UK based only.