Questions about putting batteries, solar panels, and Inverter in an older Motorhome

First- I'm still working on my systems, so I am no expert, but I have been studying this for a while and have one system up and running nicely.

With the inverter plugged in place of the generator, it will use the same electrical panels you have been using, which I assume has been working fine.
How long and what gauge will the run be from the batteries to the inverter? It is much more efficient to run the long cord from the inverter to the generator plug.
You want the charge controller and the inverter both as close as possible to the battery bank, but with the batteries vented separately away from them.

The common converter installed in most RVs will not maintain your 12 volt batteries well. Both of mine just put out 13.6 volts. Since this is a motorhome, I assume you have a 12 volt starting system. Make sure your battery isolator actually isolates. Twice now, with motorhomes, I have found failed isolators with both battery banks permanently mated. I would disconnect my house bank and see how the system functions without them. Leave the cable that went to the battery bank unconnected and insulated. If this is successful, you now understand you are working with 2 separate systems, the new 24 volt panels, charge controller, batteries, and inverter, with the old already functioning system plugging into the inverter's output.

The only part of the new system that will not readily convert between 12 and 24 volts is the inverter. Do you not already have a 12 volt inverter in the motorhome that has already been functioning? The less you change at one time, the easier troubleshooting will be, and the less likely it will be needed. used mediocre (and bad) 12 volt inverters are all over Craigslist. Ask here (in a new thread) before you buy if unsure of the make or how to test. Also, by the time the cabin is built, will there be newer, better, more fun charge controllers and 3D panels, and will you be loathe to rip a functioning system from your restored Allegro motorhome? Will you want to keep the Allegro as a guest cottage with it's own functioning solar system?

- Peter.

Hharvey- I always think better when I have a foil, I hope some of this might help you.

If you change the lighting to 24 volts, it seems you will have to leave a 24 volt system in the motorhome when you sell it.

Your charge controller will handle twice the wattage at 24 volts than at 12, but assuming that you will be purchasing more panels to support an off grid house, do you want to lock yourself into buying more 24 volt panels then? The voltage of your panels, at least to the same charge controller, do need to match. Currently the best price I'm seeing in panels- under .50 a watt- is in 1x2 meter 300ish watt, 40ish volt panels. The savings from 6 panels we put on the bus more than paid for the fancy MPPT controller.
It might be useful to think of your solar electric system as having the same evolution potential as a personal computer. You might possibly think of the system you are putting on the Allegro today as an nice little 8086 PC. By the time you have found the land, bought the land, graded the drive and house pad, built the house, dug the well, and whatever else, the 286's might be gone and the 386's or Pentiums might be out. If these terms are foreign to you, substitute wired into the car "mobile phones" evolving to the latest, greatest, Buy-Phone. Will it really be worth transferring the system from the Allegro? What new technology will be available for the house?
I would be considering the present functionality for the Allegro as the priority determiner for the system. If you can fit an extra 50% wattage using 24 volts, that is a fine argument for 24 volts. I considered switching the bus over to 24 volts, but the pre-existence of a 12 volt LED lighting system, ventilators, heater blowers, starter, and inverters made it so much more practical to stay with 12 volts.
If you haven't already, you might try a bit of CAD (cardboard aided design) to check the placement of the different panel sizes you are looking at to see how many and where you can fit them.
If you don't already have a 12 volt inverter on the motorhome, then going 24 volt will cut the amperage in half. Your existing 30 amp controller will be thrilled with that, as will the wires. If you do switch to 24 volt, there are 24 to 12 volt converters that will power your 12 volt refer and heater boards, or you can wire them to the starting battery without much draw. The heater's blower, on the other hand, does draw a lot if it is running all night. Don't forget the 12v water pump. If you use it a lot and don't drive a lot, you might not want it on the starting battery either.

If you haven't read it already, I recommend a read or two through HandyBob's rant- https://handybobsolar.wordpress.com/
I do like my Trimetric. The Pentametric is sometimes neat the same price, but I am told it is not as user friendly.

Hoping some of this is useful to you- Peter.