If you do a few overnighters and weekens to remote places, I'd seriously investigate a 3rd deep cycle battery AND a solar panel.

Also charge the deep cycle using a DC-DC charger, which isolates the Batt3 from the main batteries.  Its costly but its safe.

THe wiring is not difficult as you will see when you start to draw it up.

Essentially your existing wiring doesnt change, you just add an extra component (the DC-DC charger) to the 'house lead' from the isolating switch .....     and the Deep-cycle batt, solar panel (and regulator), Engel become a totally separate wiring loom from the output side of the DC-DC charger.

Just remember to have an isolating switch (on/off) to turn the DC-DC charger off and disconnect it from the main batteries/wiring.

 Reworded this to simplify.
Personally, (depending on your usage, outboard and boat size)

Swap one battery for an AGM or probably better a deep cycle of good capacity (AGM should really have a slightly higher charge voltage to fully charge and your outboard may not provide it)

Fit a battery charge switch (VSR) across the 1/2/both switch.

Fit small isolator between this battery and accessory panel.

If you want a solar panel it can go to the accessory supply as well however you should not run anything directly from solar with the battery isolated (which you could inadvertantly do)

This way you get

• full outboard charge rate into accessory battery once start is charged (rather than 10-20amp from a DC/DC, depending on your wallet)
• Back up starting using either "both" or accessory battery

Its just another way to do it, cheaper

Just put an Engel and solar panel in mine, ended up running seperate dual (house) batteries which runs the Engel, lights, acc etc  then another seperate lot of duals batts for the outboard, 

Recently upgraded the marine batts so had the old marine batts as my house batteries.

the 2 sets of duals are not linked so I know that the Engel and lights can be run without ever worrying about battery drain.. 

BCDC is the next step so I can charge all batts off the alternator or solar panel but for now I’m just running a solar regulator.

 I'm a sparkie, but not an auto sparkie--that's black magic stuff, nowadays--my original battery setup was two cranking batteries (860MCA ea) rigged with one as Start, one as House, separate battery switches with a Both switch between them, left off, and a VSR bridging that. The same setup you get when you buy one of thhose  integrated units,  but cheaper to put together. Thee VSR betwen them means tht you don't draw down on your Start batery when stopped, and you get full o/board charge current when the VSR closes. Well,  half outboard charge current to each battery, as they are parallelled. Fridge runs off the same House bus as all other accessories.

When I added the solar, I connected it via a solar regulator, which then fed into the House battery via an Anderson plug mounted on the battery box. So it is charging the House battery only. It seems to work fine, although i could use a larger deep cycle for House, at times. Worked fine at the Monties, except for the day we got tired of tossing lures and bait fished, moving around the shallows, anchoring frequently, short moves. And using the 12v oven for hours on end. I got a low battery warning on the electronics late in the day--remember, if you do this properly, you won't affect starting---and added up all my battery use for the day, remembering that winches power out as well as in. Went for a bit of a cruise after unplugging the fridge, didn't seem to come up as well as I thought it should. Then realised that I had also been running the live bait tank all day as well. Doh. 

So, yes, I'll change the House battery out for a larger deep cycle when it shows signs of age. The solar is only 120W, as, due to the hardtop-mounted outrigger bases, I couldn't fit bigger. 200-240 would be better. Overall the system works fine if you don't overload it--that was the only day we had any problems, and we were knocking off and shutting the motor down pretty early every day.

 Solonoid thing which wont allow you to drain your main battary

Any 4wd shop shoud have them

Ive got a start battery for each outboard and 2x 150Ah Fullriver AGMs to make a 300Ah House bank. Battery switches to start each outboard of any of the three banks if required. 

An Enerdrive DCDC 40A charger looks after the House Bank. DCDC takes the alternator voltage when the outborads are running and also has an intergrated MPPT solar regulatur so takes the voltage from a 150W solar panel when their not. Battery monitor for the house bank makes it easy to monitor current going in and out and State of Charge in %.

Works great. We run two fridges for extended offshore trips.

www.enerdrive.com.au/wp-content/uploads/2017/09/DC2DC-Plus-A4-2pp-WEB-01.pdf

good videos to watch .

https://www.youtube.com/watch?v=o_h-yoNGggM

https://www.youtube.com/user/AustralianDirect/videos

https://www.youtube.com/user/AustralianDirect/search?query=fridges