Solar panel on Ford E150 van JMC Tiara Package. Van came w own solar setup but its old and solar panel is tiny. Upgradn to 100w panel?

Can I use the cables and solar meter etc (setup van came with) to power the renogy 100w panel? Or do I need to get bigger cables etc

That depends on many factors. 100w at 12 volts is only 8 amps so the wires are relatively small, the same size you will find in a good quality extension cord, so you probably won't need to do much. Check the voltage of the panels. Years ago I did something similar and only used lights inside the van and charged the second battery from the alternator, for fresh air and to stop condensation, I cracked open a couple of windows.

If you are getting serious and looking at efficiency, you will want to put 3 100w panels in series and upgrade your charge controller. Assuming your battery can cope, that will give you enough power for a small fridge, fans, lights, charging phones and laptops. It won't be enough for your air conditioning, for that you will need to be plugged into shore power.

I'm looking at 450w myself as I would like an induction cooker and spend time in the van in spring and autumn when there's less sunlight and I use my laptop for work. But with 300w you should be perfectly ok for the summer period.