Way too much on ideas but little on actual facts, perhaps leading to lots of confusion?
One place to start is that when you are speaking of starting the RV engine, the coach batteries , charged or dead is not a factor as the engine uses the chassis or start battery to start. That combined with whether there is a method used to keep the start battery charged is a big part of the question.
The only time the coach batteries become involved in helping to start the engine is when you use the momentary switch on the dash, etc that is often called "aux, boost", or some thing like that to indicate it can be used to "boost" or jump start the engine!
Connecting or disconnecting the coach batteries will not change the engine starting in any way if the boost switch is not used. Sometimes the engine will start the second time when it didn't the first?
For maintaining the batteries there are different ways that suit different folks, so the details matter.
Many older converters would charge too high for too long and did tend to boil the water off if we did not watch carefully enough. Newer charger/converters have better systems built in to gradually reduce the voltage as the battery reaches full charge but exactly when each RV got into the better systems is not a firm answer as the idea has been around for as long as the '80's if we paid enough! So experience is one way to see if things work for you and it sounds like you are happy with your battery lifespan and doing the single jumper to get the converer charge voltage from the coach to the start battery.
Before selling our last RV, I had moved to a single wire between the positvie post and letting the converter charge both as being a very simple move as I had found the batteries were okay when done that way. Yes, it is true that the single wire WILL drain both battery sets IF the converter fails but that is one I was willing to go with to assure higher charging for both sets at all times and being less complex.
But the really basic thing to be aware of is that BOTH start and charge batteries tend to go flat, even when all the built in disconnects are used becasue there are safety items and small parts still connected in most cases.
On the start, there is a group of ignition, radio preset and other automatic items which are often still on and on the coach , there are CO and propane detectors as well to slowly drain things. The steps are often left as a safety item as they are really touchy about letting folks fall out!
If the steps are out and the light underneath is not noticed, the battery doesn't last very long!
We might look at the drawings and find what is connected or not when the switch is thrown but it is really quite simple to just flip the switch and find out what stopped working!