Do you have the roof rails? Not sure if every highlander came with them or not. I've seen ppl make showers out of PVC pipe the whole length of the roof, using either black or painted black pipe to get the water hot.
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Nemo makes some great solar shower with foot pump that are high capacity, and there are various pop-up shower shelters you could use for privacy.
For occasional use something like the showers from Nemo are probably your best bet. If you are a DIYer, you could make your own knockoff version, most likely with some 5 gallon thick plastic jerry can.
Honestly I considered a solar shower for my Overland rig, but had space to spare and got a propane one with a pump instead. I'm already carrying a cylinder for cooking, and attaching a filter to the pump lets me pull from any standing water. 12v power is pulled from the truck. Might be worth looking in to if you're already carrying propane.