First the correct term you are looking for is an EMS (Electrical Management System). There are surge guards that just that a protection against electrical surges which is about the least protection you need.
For 12 years we used a portable EMS. We never worried about theft. We did use a cable lock not for protection but to take the strain off of the plug in the receptacle.
There is an advantage to a portable. We have been in parks with some pretty questionable power posts and with the portable, we could walk down a line of posts looking for a suitable one.
The disadvantage of a portable is that the indicator is at the unit on the power pole.
A built-in EMS can have the advantage of a display inside. When we bought the Bounder, we ordered it with a built-in EMS. It has a display inside the RV but it in behind the bedroom TV, so it is not convenient.
Dale & Mark Bruss
13 Years Full-Timing Now with a 2016 Bounder 33C
40' Travel Supreme winter residence
Lots of RV Information at www.dmbruss.com