There are inverter-charger combinations that include the changeover relay to go from shore to inverter power. Typically these are for larger RV's and they used to be horribly expensive. You should note that once you cross into any installed 110 volt circuit even in a vehicle you have to consider "code" requirements including that all devices and components be designed and listed for the application. In the Westy we can get effected both as a recreational vehicle and a dwelling. 

That said take a look at the Xantrex Freedom  series of inverter-charger combinations. 

The Freedom HF Inverter/Charger is one of the smallest, lightest, and most affordable inverter/chargers in the market today. The Freedom HF Inverter/Charger features modified sine wave output and is designed to operate entertainment systems and small appliances in boats, RVs and trucks. The Freedom HF is available in three models 1000 watts with a 20-amp charger, 1000 watts with a 55-amp charger and 1800 watts with a 40-amp charger. - See more at: http://invertersupply.com/index.php?main_page=product_info&products_id=1464&cPath=0_245#sthash.9wFFOfC6.dpuf

Some more information

Product Features Designed to power demanding loads such as microwave ovens, TVs, DVD players, small appliances and other electronic equipment 
Removable digital display pre-attached with 25 extension cable for remote system monitoring 
Ignition lockout feature helps to minimize battery drain 
Automatic three-stage battery charger 
Designed to be hardwired using quick-connect AC terminal blocks or by connecting AC through GFCI receptacles (included) 
Built-in 30-amp relay to automatically transfer between inverter power and incoming AC utility power 
Product Specifications | Download
Product Manual | Download
Protection Features Battery over-voltage and under-voltage protection 
Over-temperature shutdown 
Automatic overload protection 
Short-circuit protection 
Ground-fault protection 
ETL certified to UL458 for use in mobile RV and commercial applications 
- See more at: http://invertersupply.com/index.php?main_page=product_info&products_id=1464&cPath=0_245#sthash.9wFFOfC6.dpuf

Do some shopping as prices vary. The unit with the 20 amp power supply-charger is ideal for our smaller batteries. With an automatic relay such as the sure power this will keep both chassis and house batteries in shape during storage. 

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Edward Maglott
Sent: Tuesday, August 25, 2015 10:43 AM
To: vanagon@GERRY.VANAGON.COM
Subject: automatic aux battery charger + inverter setup

I had a homemade setup in my van that I need to replace and would like to improve.  I had a switch in the van mounted down near the outlet by the bed.  In one position it connected all the AC outlets to shore power.  In the other it connected them all to an inverter and turned the inverter on.
I relied on discipline to not switch it to the inverter position when plugged into shore power.  That aspect of the system was obviously far from ideal.  If I wanted to use a charger, I would plug it in manually and connect it to the aux battery and again, make sure I did not have the switch in the "inverter position."

So what I would like is:
-A system that would start charging the aux battery when I was plugged into shore power.  2 Amp maximum power is fine.
-When not plugged into shore power I would have the option to power all AC outlets from inverter.  Inverter capacity of 400W should be fine.
-Not blow up or cause damage if I accidentally turn on the inverter when plugged into shore power.

Is there a commercially available device to do this?  I hear you volks talk about using some "surepower" device or something like that?

Thanks,
Edward