2 6v parallel with 1 12v

X2 MrWiz...it will be fine for camping. When at home charge the 12 and the 6's separately.

The problem is that the charging unit is not going to take all the batteries to 100% charge level. That may not be important and a 90% charge level may be OK. On the other hand if the controller overcharges one or more of the batteries there will be water loss. and if the former I would carefully monitor the water level in the batteries and even carry along a small container with distilled water. I would also want to be sure that all were the same type, either all flooded lead acid type or all AGM.

Keep the 12V as a spare....the 6V batteries will do you well, and you won't have the issues that might come up, with the system trying to draw from different size units.
Will it work? Yeah, but it's always best to try and balance things out....same as getting the same tires. Can you run with that little space saver...yes, but it's not the best way to go!

Just think like this:

When properly installed in an RV (Motor home or trailer) There are NO SIX VOLT BATTERIES,, Only 12 volts that come in 2 pieces for ease of handling.

So you want to hook up like this

{Bat}t{ery}
-(Battery}+

And yes, that will work I saw posts about efficiency and other yammer warnings, For the most part, those folks do not know what they are talking about. They will work fine.

There is, however, one slight danger.

should the 12 volt short out (Shorted cell) there is enough power in the six volt pair to make it go "pop" and spray acid all over the place.

yes it can be done
not the perfect situation, but acceptable with 'low' amp loads
lights, fan, water pump, etc..

using an inverter with high wattage loads, is where the difference in battery design and age will have a more noticeable effect

It's not ideal but it will work.

All shall be revealed: The 12 Volt Side of Life

It will work but not be efficient. For maximum efficiency all batteries used in parallel should be the same size, power and age. When any of those things are mismatched efficiency drops. Charging may be incomplete and they will discharge faster than optimum. Weaker batteries in the group will drain the stronger ones down to their level.
Good luck / Skip

I've never done it, but I would certainly want the two 6v in series and the one 12v to have the same voltage reading, or as near the same as you can get as measured by a good digital voltmeter.