How to tell if your convertor is working correctly?

Is there a way the average guy can tell if his camper electrical convertor is actually charging the batteries?    The reason I ask, is throughout the winter I put the battery charger (battery tender type) on the camper batteries, and my particular tender shows at what state the batteries are in.  They often are in the 75% range on the tender meter when I put it on, and it takes a day or two for them to be fully up to 100%.  Some times 3 or 4 days. 
It makes me wonder why the camper electrical convertor on my 2010 Arctic Fox camper doesn't do the job of bringing them to 100%?  I'm a newbie when it comes to electricity and components with regard to knowledge on the subject, but I do like to keep my maintenance upgraded.
One thing I do is...in having dual batteries in my camper, I disconnect the terminals prior to charging, as I've tried in the past to charge two batteries in my pickup, and found it works much more efficiently by doing one at a time, thus have done the same with the camper.  Is there a 'connection' of some kind that I could purchase, where instead of disconnecting the terminals, I could just flip a switch to disconnect power from the two batteries?

While on the subject, I've just recently put together a 'new to me' truck, with the camper, and I'm wondering if there is an isolator (if that's the right term) in the system, where when the truck is running that it charges the camper batteries?  How would one figure out if that is the case?

Battery tenders are designed to maintain battery charge, not efficiently recharge them so the time it is taking may be normal.

You should not have to disconnect the batteries to charge them. The easiest way to test the converter/charger is to put a digital voltmeter across the battery terminals with the charger on and charger off and check voltage. You should see 12.8 volts or so after the batteries have set for awhile without a charge and should see between 13.2 and 14.2 with the charger on depending on the charge state of the batteries.

You can buy a simple manual knife blade type battery disconnect to connect to the battery post to isolate the batteries or a remote controlled relay to do the same job.

A battery isolator on your truck will insure you do not run down the truck battery while parked yet be able to charge the batteries when the truck engine is running. Check the voltage on the camper battery while the truck is running. You should see a charging voltage of 13.2 volts or more. Shut the truck off, put a load on the camper battery and see if the truck battery voltage stays up as the camper battery draws down. If it does you have an isolator in the system.

The battery charger/tender that I use is of the type to eliminate 'Sulfation' such as in this site - http://batteryminders.com/sulfation.php which brings the batteries up to a full 100% charge.  I'll eventually get a solar panel to help keep things charged for those long periods of stoppage, although I do have the Onan on board generator as well, but find it a little annoying if left on for longer periods.   
Thanks for the post, much appreciated!