Lots of different ways to deal with keeping both sets of batteries charged during storage! One I like as it cuts the extra equipment needed and makes it simple to hook up, is tying the two together.
If you have been driving the RV, there is a mode solenoid setup which connects coach to chassis so that the engine alternator is putting "some" charge into the coach batteries as we drive.
If we have been using the coach battery while camping and not hooked to power, they may be somewhat run down, so it is a cheap and simple way to get a little put back in them if we drive far enough! It is a slow process that may take at least four hours for a full charge, but "some"!
When we get back home the difference in voltages of the two groups is near zero as both have been connected together.
That does a couple good things for us when we want to finish off or add charge to either.
Since the voltages of each are so near the same right at the posts, the wire we use to tie them together doesn't have to be very large! For jumper cables and such we use really large but in this case, that is not needed and totally safe, assuming we watch a small point.
The size of wire is set by how much current we want it to carry. That current is determined by the "difference in potential' of the two points or difference in voltage for the way most of us think? When we drive the voltage is going to be the same or "no difference" so the current flowing between them if we put a jumper from one to the other will be very low!
We can get by very well if we just clip some smallish wire like a number 12 gauge between the two positive battery posts. We don't need a second wire for the jumper as the negative posts both go to ground already! In a house, we use 12 gauge to carry 20 amps but that is way, way more than we will get in this use! A small wire with large clips to fit on a battery cable some way?
In your case, the batteries for starting and for coach are side by side under the steps, I think?
So if you drive the RV and the two groups have been connected together, if we open the step and strap the two positives together within an hour or so, there will be hardly any difference from what was done as we drive!
The converter that is working when we are plugged in and the battery cutoff switch is left on, will charge both.
We can save the extra charger and connecting them in harder methods but with one caution!
The caution is that if we forget to take this strap off and go out camping without power, we can run BOTH start and coach batteries down to strand ourselves!
Maybe a note on the steering wheel is a good idea until we get the habit?