Truck charge camper battery?

The so called charging capability of the truck camper being connected to the truck is not going to charge the batteries in a meaningful way. Yes it will charge while driving via the alternator, but besides the alternator output variable, the wire run from the trucks electrical to the truck camper is too long for the wire size used to get sufficient battery charging.

I have a hundred watt solar panel on my roof also, and I will run my frig on DC power. But you have to watch your battery's if they get a little low just turn it off for 2 hours and the frig will stay cold. I run the frig on DC only when it is to windy to keep a flame going on propane. Hope this helps.

It would depend on the capacity of the truck's alternator. Unlike other camper brands, your Lance camper has a factory supplied proprietary heavy duty power supply wire built into the umbilical cord to the truck. The purpose of the thicker DC power supply wire is to keep the camper battery charged while driving.

That said, I don't know the answer to your question. . .there are too many variables (such as 'is your truck equipped with dual alternators?').
However, the fridge running on DC power is a power hog. It pulls DC power like crazy. I have doubts as to the ability of the truck generator to supply enough power to actually charge the camper battery while driving.

My suspicion is that the truck alternator is more of a "life support" for the camper battery while it dies a slow death from the Fridge power drain.

If you forget to switch to propane at your destination arrival, your camper battery will be dead in a very short time.

(edited for electrical power generation source)

On 12 volts DC, my dometic in my Lance takes 15 amps, while on 110 volts, it only takes 1.6 amps.

If you have the typical Lance plug, and a battery isolator in the camper, the battery isolator won't let the truck battery get below a certain voltage, so your truck can start. Battery isolators, however, have been known to fail over time, or have marginal voltage settings, thus draining your truck battery and not allowing you to start the truck after a while.

That size solar panel might be marginal, depending on the sunlight available, to keep the camper battery charged.

If you have one battery on the camper, like I do, the 100 amp-hour battery on mine will work for 2-3 hours with the camper unplugged, but eventually it will be drained by the fridge, depending on how much the fridge compressor works.

The truck charges it while driving.

I'm not sure how many batteries the Lance 921 has. If it has two with your 100 watt solar panel, it might be enough for a whole day unplugged.

I'm planning on adding some solar too, but I'm thinking along the lines of 200 watts of solar, which is about 16-ish amps at 12 volts. I dont think a 100 watt panel is enough.

If your solar is roof mounted (not a suitcase) it's going to be working while your travelling so you should be fine.

Maybe but probably not. Depends on alternator capacity. Unless you have a particularly beefy alternator, the fridge is likely to draw more than the truck can provide. On the other hand, if it's been working for you, it evidently doesn't overtax your house battery.