You didn't mention the make and model of the furnace. Generators (gas and LP) often have carburetor settings for high altitude camping. They do generate less power at altitude, there are calculations that can show how much.
The recommendation to replace the LP regulator might help,
IF you replaced it with one that can be adjusted easily. Did either shop use a manometer to measure the gas pressure at the furnace? Typically at lower elevations, most gas appliances want 11 inches of water column (WC) Because of reduced air pressure and oxygen at altitude, it' suggested to reduce the gas pressure to as low as 5" WC. You could also make sure the mixing tube, where air and gas mix right before the ignitor, is clean and free of cobwebs, dirt, etc.
Other suggestions are to exchange with a smaller jet so the gas is restricted at the orifice. I saw one suggestion to take a single strand of copper stranded wire, bend a hook in the end so you don't lose it in the jet, and insert it into the orifice to reduce its size at altitude. That could easily be removed when you come out of the clouds.
Many folks report no issues camping at higher altitudes, others have trouble as you report. I've personally camped from sea level to 10,000 ft (Leadville, CO) without problems, water heater, furnace, stove, and refrigerator worked fine. Just lucky I guess.
Some appliance manuals suggest you don't try to use LP mode of the refrigerator above 6,500', 120v electric only. That limits you to electric campsites.
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