Ted,
The simplest thing you can do is install a manual battery selector switch of the type you'll see in boats that have more than one battery...they're cheap and do the job, but you have to do yours and remember to switch to combine when driving and separate when camped (so you don't run your starting battery down). The next option would be, as was previously mentioned, to install a power relay that would energize when your engine is running (or ignition is switched on) and combine the two batteries for charging while underway. Obviously, this takes you having to remember to operate the manual switch out of the equation.
In either of the two cases above, your solar panel/charge controller would only be connected to your Auxiliary battery and can be considered separately.
A key aspect to this, as luthj mentioned, is that you need to determine what the charge voltage is for your vehicle and size the wiring from your starting battery to your aux battery appropriately. Appropriately meaning large enough to not only handle the amperage when charging, but also to have low enough resistance that the voltage drop in the cable when charging isn't so great as to slow your charge rate to next to nothing. In my case, with a 2013 Silverado 2500HD (diesel), my charge voltage is typically around 14.4v, but I hear that a lot of trucks max out in the 13.8V range. You need to measure yours before you go much further.
I ran 6ga wires for both pos (fused) and neg from one of my starting batteries (truck has two, being a diesel) to the power jack in the front of the truck bed where the camper plugs in. From there, inside the camper, the factory wiring is 8ga to the rear of the camper where the battery is located. Charging while driving took a while due to the voltage drop in the wiring when the battery was drawing the heaviest (highest amperage) charging load. To make matters worse, while traveling, I pretty much had to run my original Norcold refrigerator off DC instead of LP...and on DC, it drew more than 10A pretty much continuously...further dropping the voltage/current available for charging the battery. Now I have a compressor based refrigerator that only draws 3.7A on a 30% duty cycle under normal usage, more like 25% overnight when it's not being opened...and a 100Ah Lithium battery instead of the lead-acid battery that was standard. I now have 2x the useable power and it charges much faster, whether from the truck, solar or the camper internal charger when on shore power.
As for solar, a lot will depend on your camping scenarios...how long will you be in one place, are you in desert, forest, do you park in the shade if available, is it predominantly sunny or cloudy. Unless you can get 4-5 hours of clear sun per day, 100watts of solar will probably not replace the power your fridge uses in a day. Mine typically uses 300-450 watt/hours per day, depending on ambient temps and how often I'm in it. A 100W panel with a decent MPPT controller, clear skies with full sun for the whole day might get you 500 watt hours of production into your battery, depending on where you are, geographically and time of year. On a mostly cloudy day, you'd be lucky to get 20% of that. If your truck is in the shade with a panel mounted on it...same thing, 5% to 10% of max production. For that reason, I only have one panel mounted on my camper roof and 3 available to sit out in the sun on an extension cord if I have a good shady place to park and a sunny spot available. If I'm parked in the sun, I'll just put them all up on the roof until I move again. I like to keep my options open!
Lot's to consider, but a key is having some DATA...knowing what your loads are, your usage and planning accordingly.
