Correct wiring and battery set up advice needed

[the evil twin]

I have made a few guesses about your setup in coming up with the following suggestions
The main factors you mention for sure are-
That you have an Oven and Sine Wave Inverter (both use high current when they are running)
That a DC-DC charger is not in the budget at the moment.
A 90 amp Alternator is used (not much advantage in a bigger one anyway really)
You have 180 watts of Solar for camping that must have some sort of Reg already
Not sure what you have for the 40

Sooo,
I would do the following
-keep the basic Isolator under the bonnet
-in the existing leads that supply the rear (3rd) battery disconnect the battery and fit an Anderson Power Pole or mini Anderson or similar plug.
-remove the rear battery and for whenever you need it make a small fly lead for it that plugs into the Anderson.
-keep the existing power distribution to accessories in the rear however you have it set up so it runs on the under bonnet aux if the 3rd battery isn't fitted.
So far you have spent about $10
- not sure what you have with the 40 or what you intend but if it is going on the vehicle permanent then a cheaper PV reg will be all you need for floating the batteries
Maybe $30 so that is $40
- keep the 3rd battery charged up at home on a decent mains or solar charger (use the 40 panel and reg if it isn't on the truck) every week or so
- regularly (once a monthish) and after any major use of the Aux put a mains charger on the vehicle for 24 hours or so
- if it is feasible and you can use one reg and swap it between the 40 and the 180 panels then even better.

Remember, a Solar Panel and Reg is a DC/DC charger so use it whenever it is available

Advantages to you
Cost
Simplicity
Versatility
The aux's will charge quicker to 80%-85% off the Alternator than a DC/DC if they are well down on charge

Disadvantages to you
You need to remember to isolate the Cranker and reconnect the Aux/s
You may lose a small amount of battery capacity (maybe 10% max) in some circumstances of low solar availability and higher power useage

Hope that helps