rv won't start, need to jump
First, nearly all motorhomes have an "emergency start" switch on the dash. If you find it you can press it and hold it while you start the chassis engine to use the coach batteries as a source for the jump start. What it does is to by-pass the isolator that keeps the coach from running down the chassis battery or the other way around. It is designed to do this and will harm nothing. If the coach batteries are up, that is the best and easiest way to do this.
Another thing you might want to do is to first start the genset and allow it to warm up. Once it has started, hold the "emergency start" switch in the emergency position for a few minutes to allow the converter to supply current to the chassis battery before you try the starter. Doing that will not only boost the chassis battery a little, but the current supplied from the genset, via the converter, will warm all of the batteries a little. A warm battery will suppy far more power than can a very cold one. Very likely, this will be plenty to start the chassis engine.
If the genset will not start either and the the emergency start does not do the job, then you will have to jump start, or get a battery charger. Which ever you use, there are no special precautions you need to take since the chassis battery is isolated from the coach loads when the chassis engine is not running and the "emergency start" switch is in it's normal position. I would connect the source vehicle with it's engine running and I would allow it to charge the chassis battery for at least a minute or two before attempting to start the engine. Once that is done, allow the battery to warm and then check to be sure that it did not freeze and crack, and assuming it didn't(which it probably did not), check the electrolite levels and add distilled water as needed.
One other thing to note. If you ever find that the genset will not start, but the chassis engine will, once it is started there is no problem in using that "emergency start" switch to cross connect and thus jump start the genset as well. It will work either way.
