Best way to supplement battery bank?

The scenario all depends upon what you power consumption is over each 24 hour period.

Hypothetically your asking your Trojans to last 5-days & the 12 volts to last 5-days when each set is connected to the RV & solar. I believe you will end up going below 50% SOC for each set when in use in that time frame. I don't think they will last that long.

I'd just disconnect the Trojans & connect the 12's. My leads are long enough that they will reach batteries set on the ground underneath the tongue, or set them on something that raises them high enough to accomplish this.

Borrow a generator & portable battery charger to re-charge the bank not in use.

Take a volt meter with you.

Or, just leave the 12-volts at home & use the generator in conjunction with solar to keep the Trojans up.

JMO

Once parked and when you think you may need extra AmpHours I'd use a good set of jumper cables (most are not very good) to add another battery to your system.

Connecting with jumper cables is not a good idea since a good connection can't be guaranteed. Either swap out the batteries if you need to or carry a dedicated set of cable and connect the second set to the coach batteries positive to positive and negative to negative. This will give toy one larger battery bank and your solar panel, alternator, etc. will still charge everything. You could even let them sit under the coach in battery boxes but beware of theft. Chuck