Feeling that you can't stop is NOT normal in any size trailer.
I personally feel that from your description, your brakes are not up to snuff. Have you adjusted them?
Be wary when an RV dealer says they "checked" or "inspected" something. Normally that is a simple visual inspection, and if it isn't falling apart or falling off, it's deemed "good." A true check would involve pulling the hubs and drums to make sure the wheel bearing grease hasn't leaked into the brake drum, to see if there is adequate friction material left on the shoe, to see if the drums are not overly worn, and a proper adjustment upon reassembly.