As mentioned you alt is never going to directly sense what voltage is getting to your camper battery, it seeks to maintain the vehicle battery. However when you draw off the vehicle battery to charge your camper battery it should pull amps and draw down the vehicle battery voltage causing the alt. to increase output as needed.
It'd check the voltage along your wiring to see where you get the drop off (ie does it happen mid run, across the surepower, or just at the battery?) because that just seems way to low. A loose/corroded connection or such increasing resistance could be your culprit.
I like the DC-DC charger because it will make up for some voltage drop as well as charging the battery at the correct bulk and float rates for my AGM batteries and ideally longer life than just giving them whatever the alt throws out (looking through some notes it is supposed to bulk charge at 13.7V, over charge at 14.4V and float at 13.2V). However I still think your getting too much drop in general for some reason that really should be addressed/investigated.
Finally got some time to investigate and found the culprit...the positive wire on the marinco plug was loose and had some corrosion. Cleaned it up, reconnected and back to charging! Thanks again for all the help! Cheers!
