It's cos(your latitude-23.44°), or just 1 in the tropics, unless I'm misunderstanding the question. I assume this is for a panel mounted level? Make sure your calculator isn't expecting radians!

If you mean ideal at a specific time and date, you can use the formula from the Wikipedia page. Substituting in 23.44°*sin(years since last March equinox*360°) for the (rough) solar declination, and longitude+UTC for hour angle, you get:

sin(latitude north) * sin(23.44°*sin(year fraction*360°))  +  cos(latitude north) * cos(23.44°*sin(year fraction*360°)) * cos(longitude+UTC time). 

If you're just looking to avoid writing that out every time, I'll happily implement it in the common programming language of your choice. If you really care about the deviation caused by the elliptical orbit of the Earth you'll need more complicated math.

Edit: That should be signed latitude on the second equation, and March equinox, not solstice. Fixed.

Is there a good online tool for calculating the cosign of solar Zenith

There are an online calculators like this one from NOAA. This fork adds the ability to update the time with a click rather than manually. There are others but I haven't played with them much.

Since I travel constantly I've been on casual lookout for an app that does the calcs for us based on local time / position but I haven't seen any. Several apps show the solar zenith angle and we can take the COS of that manually with a calculator with trig functions. The standard android calculator app will do it.

so I can know how much solar my my panels could be making in ideal conditions?

Yeah, it's an imperfect tool for our purposes but better than nothing. Combining it with data from a solar irradiance meter would be great but right now I can't justify ~$100 to devote to the cause. :-)

I've found these Android apps useful:

• Trail Sense and it's Solar Panel Aligner tool (given a time length or all day, calculate and assist alignment of panels to maximize production)
• RV Solar Forecast to see theoretical maximum possible
• Sun Surveyor to see where I can put my portable panels to get the maximum light without shading

I put it under a secondary cover and used it for my outside DC outlet, and replaced my external solar panel outlet with a CNLINKO LP-28 2-Pin IP67 Waterproof Connector rated up to 50A.

That connector looks interesting. It addresses one of my unstated concerns, the panel wires pulling down on the connector. This one looks like it has enough "neck" to relieve the strain. If I were doing the van again I'd mount all the inlets in one place under a secondary cover as you describe.