If it were me I would invest my money in a Black&Decker VEC1093DBD smart-mode technology 40AMP portable Battery Charger.
This is something you will need to support your deep cycle batteries for years to come. Has all of the features to keep your batteries in great shape and long life.
It also has a 110AMP Battery start assist feature.
It is my opinion you should be using your on-board batteries to operate all of your 12VDC items with good operating batteries. If the batteries are run down and not performing like they should then they are not being taken care of properly. My trailer has the smart-mode technology converter/charger unit being used which really almost takes care of my batteries automatically.
I can't recall when I have had a run down battery... We do most of our camping off the power grid and just like in the ole cowboys days the horse gets taken care of first haha... Same with the batteries. Our camping game plan is to be able to have all of the 120VAC and 12VDC toys we want to have with us when camping off the power grid and have enough battery power to operate them for one day/night camping experience. Then at 8Am the next morning we will connect our 30AMP trailer directly to the 2KW Honda Generator 120VAC receptacle using a RV30A-15A long adapter and recharge our batteries back up to their 90% charge state in as little as three hours time using our on-board smart mode charging technology. We try to never let our batteries get discharged below 12.0VDC which is somewhere around 50% state of charge. We can do a good 10-12 days of these 50% to 90% charge cycles on our batteries until we need to do a full 100% charge which will take around 12 hours of generator run time. This is the point where the batteries will start internal destruction if not re-charged back to a full 100% charge state. Since most public camp grounds wont let you run a generator for 12 hours continuous here on the East side of the US this is usually when we head for the house.
My bigger point here is I don't think I would want to put a big large unregulated current generating device that is capable of producing 200AMPs or more etc across my battery terminals to do a so called BOOST. Who knows what the DC Voltage output is when in this mode. That has got to hard on those batteries.... This would also be presented to the trailer Power Distribution Panel and could cause problems with some of your trailer systems and electronics. This would be flirting with disaster to me... I would not use the 110AMPS jump start from my B&D 40AMP battery charger as well. Just my opinion mind you... Just something I would not want to do. I get nervous with my systems and electronics when my converter/charger unit is at the 14.4VDC charging level.
I think if I did do something like this I would definitely disconnect my batteries from the power distribution panel with my battery disconnect switches if my goal was to just boost the DC power to operate the trailer slides.
Just my thoughts...