Under hood cleaning, how often and how
 

Was wondering how many times do you clean your engine and engine bay and how do you do it.
Here in AZ it gets dusty fast. That's sand dust eats parts.

i powerwash but make sure to stay away from the alternator, fusebox, and under modules and electronics. The spots i do wash, i move quickly and keep the wand far enough away so the pressure is low.
The fuse box covers on new vehiclec have a picture of a pressure washer wand and a big X across it.

I must Simple Green over everything and then use a garden hose with a nozzle and rinse it off. Then I take a towel and dry things that are easy to reach.

I don't own a pressure washer, so I go to the coin-operated car wash. I use the ambient pressure on the rinse setting. I never pull the trigger for full power when hosing out the engine bay. I do it a few times a year.

I generally rinse mine every time I was the truck, I too use Simple Green in a spray bottle. Just like the vehicle thou you have to get in the engine bay with a sponge or cloth to hit the surface areas. I don't use the pressure washer but I do use the hose end sprayer with the jet for the firewall and frame areas... I use a leaf blower to dry out the bay , use armor-all on the plastics and WD-40 on the metal areas... My 07 still looks showroom new under the hood :D

This.

Does your Aux battery have side terminals? Couldn't see any connection.

Ah very good observation, but no, no side terminals. the battery as picture was in fact not connected at the time.
I leave the positive connected as it's more cumbersome to disconnect and secure from shorting unless I disconnect it from both ends, then I have a length of wire to deal with and loose...
So what I do from time to time is disconnect the negative cable and tuck it away, I do this to do a sanity check for the main battery, I have had the main go bad and it would recover itself by drawing on the aux as I do not have mine isolated ( the aux is the camper battery and I keep it conditioned and as a back up in the truck.)
I will either charge it with the battery charger or from time to time connect the negative cable for a week or so. I always use the aux battery if someone needs a jump to protect my charging system. I don't have the aux battery setup for starting, but it does backfeed into the fuse box for additional power for accessories with a 40amp fuse in place. In theory, when connected to the system it could act as additional starting power, but the 40 amp fuse will blow if too much current is drawn, I've had it happen with a failing main battery.