If you’re running a steady speed downhill the brakes won’t engage, only when braking or downshifting which is what you want them to do. If your trailer brakes don’t engage then your RV will be doing the braking for both vehicles which will be an extra load that will cause overheating of the RV brakes. Just because you’re going downhill doesn’t mean the dolly brakes will automatically engage, they need a stopping force on the RV. No offense please, but you’re over thinking this. The surge brakes are meant to slow the tow dolly and take the load off the RV brakes.
If you have electric brakes they will engage on a downhill when you apply the RV brakes and stay engaged until you take your foot off the brake. If you only downshift an electric brake dolly will now start to “push” the RV because they have no signal to activate them, again, more load on the RV.
Surge brakes mimic the actions of the RV, you brake, they brake to keep the braking loads more equal. Perhaps a trip to a “good” dealer would be helpful, they could point out the differences and assist you with your decision. Just make sure whoever you talk to has actual experience with both types of dollies.
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