Daisy chain cable size?

You will get less voltage drop, hence more power, with 1g cables. If you have the option, always go with larger cables. Will the 4g cables work acceptably? Yes.

4 ga is good for 55 amps and up depending on the type wire you use. Furnace and lights will not test that at all.

I assume you are paralleling multiple 12 volt batteries.

Assuming 10' one way and 15A load 4 ga is MORE than adequate.

Best way would be to used balanced wiring.

http://www.smartgauge.co.uk/batt_con.html

#4 all day long.

https://www.customcableusa.com/products/custom-battery-cable-marine-grade-by-the-foot

The bigger the cable the better the cable the cables are bigger at WELDERS ARE US

Ideally you use the same size as the main cable from the battery to the rest of the RV. or bigger.

For parallel I might "Cheat" depending on what's on hand
But Ideally you use the same size as the main cable into the RV.. Or bigger.

While bigger is better in principal unless you are pulling huge amperage, the improvement is likely to be negligible.

If it's just lights, water pump and maybe a furnace fan, the smaller cables are just fine.

Use a voltage drop calculator rather than speculation. Easy google solution.
You’ll likely find that you only really need 6 or 8 ga depending on actual lengths if you can keep it relatively short.
If building a permanent system I’d likely upsize a bit more but frankly, you could just as easily round up a few sets of jumper cables even cheapo ones and achieve the same or better results for free.

If in doubt, larger gauge is better.

Can't believe there is enough difference in cost that you would go with smaller gauge wire.

small detail: do not daisy chain a bunch of batteries. Doing so will cause them to carry uneven loads, ware out unevenly and make headaches down the line. If you have a "bunch" of them, connect them all to a single common connection point using carefully measured cables of matched lengths. To be sure, this will have the closest batteries with excessively long cables (or so it will appear), but the resistance will match, they will carry the load equally, and work much better in the long run.