Fishpants, ya beat me to it!
Your house bank being four batteries should stand up fine to overnight electrical draw unless they need replacement or you’re really pulling on them all night.
To help decide if replacement is where to begin check if they are into or past their fifth year of service then put an amp meter in line and turn on everything you ran that night to reveal the amp draw during that night. Fully charge the batteries if they will do that, shut off everything you can do without at night, turn the fridge down and check the new, lower amp draw then see if the current batteries are toast after overnighting with the new, lower amp draw..
If the four house batteries show they can stand up well to the new, lower amp draw overnight you could consider keeping them and adding the thruster battery, there’s threads on the “Nuts about that and I did that in my old R25. If they are anywhere around 4 years or older and can’t handle the new low amp draw overnight fresh batteries are indicated and replace the thruster battery while you're at it so you can very confidently add it to the house bank.
When sorting out batteries issues keep in the forefront of your thoughts that, while there’s all sorts of ways to keep an eye on new batteries as their service life goes on and all sorts of ways to help make that service life as long as it can be; there’s really really no way to bring them back once they can’t do what’s needed of them.
Batteries tend to work best together if they are all the same age, same type, same amperage hour, same style, same brand, same state of health, get charged the same etc. etc. etc. You might also consider a solar panel which will provide a moderate amp charge all day every sunny day because fully charged batteries are happy batteries!
Once your sorted out, to keep an eye on things you can check standing voltage with the built in gauge when you retire and when you awake, this will give you an inkling into how you’re doing on the overnight draw and the batteries capabilities as they diminish over time. The important number here is the morning number, that’s their state when they’re done doing what you need them to do time and again for years. I write information like this down somewhere because I can’t remember breakfast.
Use the same gauge to check standing voltage of your new starting and thruster batteries under load by putting the gauge on that battery and noting the voltage while you operate the starter and thruster.
I’ve constantly owned a truck, a car, a UTV, a camper and boats for the last 25 or so years so I’m operating, maintaining and replacing eleven batteries all the time which means the above advice was learned the hard way.
Use a tremendous amount of caution and safety gear when working with batteries, think of them as powerful, eager to short out, explosive gas making, acid filled little bombs because that’s what they are. I cover all my flesh completely and wear a full face shield and safety goggles when I have anything to do with them.
Also, forget using any spring loaded clip on style connector to carry electricity, those jumper cables you mentioned. When any one of the four connectors pops off you will be what’s known in boating circles as “on fire.”
Hopes this helps
