No matter the device they will give you bad directions at some point so knowing the route you want to travel before you leave is most important. Beyond that, Google Maps and WAZE will give you the most currect road conditions and travel directions. They do not provide height and weight detours. I tried a trucker GPS but it would also keep us off roads where no trucks were allowed but would have been fine (and allowed) for RV's.
I'm not sure what you mean by "safest route for the size...". You're not pulling a 65-foot long, 20-foot wide, 20-foot tall ranch home on wheels. It's just a trailer, there's thousands of them on the road, most bigger than yours. If you see trucks on the road, you're fine.