Downshifting on the downhill side is good if done correctly. From a safety standpoint, the more you can slow down without using your brakes, the less likely you are to experience "brake fade". There's a lot of variables, wet/dry road, how steep, speed limit, what's at the bottom, other traffic, etc. Basically start the downhill as slow as you can, ease off the throttle, maybe even use the brakes a bit before you start picking up speed. When it begins to pick up speed, downshift. Don't wait too long to downshift, you want to shift into a reasonable RPM range of the engine, you don't want it screaming at maximum RPM and risking a blown engine. You will probably continue to pick up more speed, start using your brakes to control your speed. Don't let the speed build up, even if traffic behind you gets upset. You may want to apply and release the brakes in a rhythm, say 3 or 4 seconds on, 3 or 4 seconds off. This helps keep them a little cooler. Upshift if the engine revs too high and use more brakes. Keep an eye on the temperature gauge, and note how the brakes feel. Consider pulling off to the shoulder if it is starting to overheat or the brakes are starting to fade. And plan on filling the trailer water tank when you reach your destination rather than hauling a lot of water up and down the hills.
I can identify with the questions you're asking because it wasn't that long ago I had the same questions. In Ontario, the maximum grade allowed on a highway is 12%, twice as steep as allowed on interstate highways. There is one stretch along Lake Superior that is about 3 miles downhill with a sharp bend at the bottom and Lake Superior sixty feet below on the other side of the guardrail. White knuckles all the way, but starting slow (40-45 mph) and not letting the speed build up it's no problem.
Chrysler has known for 30 years that people buy their minivans and then tow trailers all over North America with them. Your T/C should handle it very well if it is in good mechanical condition (agree with mickrock's post). Very wise to ask questions first, but listen to what the T/C is telling you, watch your speed, think ahead, and you'll be fine. And you said you worry about driving alone... if that is because of the possibility of breakdowns, look up Coachnet...roadside service specifically for folks with trailers or RV's. Have a great trip!