IN CONCLUSION: Info provided by Steve, Scott, Theo, kaube, others was spot-on, many thanks to all.
On Friday I drove south thru the CA central valley, then up to the Mojave high desert, thru Boron, south to Apple Valley, and finally that evening, over the backside route into Big Bear Lake, ~ 450 miles. The climb from Apple Valley was ~4,000 feet over ~40 miles but mostly a straight run with just a short distance in tight curves or switchbacks (as Capt Steve reported). While much higher in elevation, it offers a route no worse than towing to Lake Berryessa in Napa Valley (zero shoulders, but consistently 10 or 11 foot lanes). My Sierra pickup, 5.3L V-8, would pull the Mac up there with just a bit of sweat. Monday's downhill run was the frontside route (CA-18 / CA-30) into San Bernardino. About the same 40 miles long, but all of it a very curvy road, albeit with generally longer-radiused curves. Both routes have a short stretch of very tight switchbacks near their summits.
In a towing overview, starting from my son's location, south along the Corona freeway:
- - would it be tough to exchange the direct 40 miles of frontside curves up to 8.000 ft summit
(8,000 ft in just 40 miles);
..... for the alternative .....
- adding an extra 45 miles of a straight freeway climb, 4,000 feet into Apple Valley,
plus the remaining 40 miles of fairly straight climb over the backside summit 
Hmmmm .... 8,000 ft over 85 miles is a much more gradual climb, halving the ascent rate, relatively straighter with many fewer curves. My son towed a 4,000 lb. Taurus travel trailer up the frontside this past weekend. Just guessing that he might still go directly up the frontside route. But even if starting from his place, I think I might choose to double the distance and halve the ascent-rate for towing my 26X to Big Bear. It seems easier on the equipment, and easier on me!
About Holloway's campground, a bit pricey at $45 per night, but clean and well maintained. I rented one of their pontoon boats for a half-day (~$50/hour) and we saw the whole lake. Fish were feeding at the surface from 5 to 7 pm, almost jumping into the boat. Unfortunately, my grandson's lures were completely unappetizing ($20 for a 2-day fishing license).
About Big Bear Lake ... it's an absolutely beautiful place and I think the lake is amply sized to be fun for a week. We had ~90 degrees and 10+ knot winds for our 2 days and it's definitely big enough to fall asleep during a single tack ... though only sailboats we saw were under 20 footers, or Hobies. The surrounding peaks aren't as high above lake level as Tahoe ... can't really judge how they were affecting the wind. Based on last weekend's experience with lake full to the brim, I would not hesitate to tow my 26X up there to spend a few days sailing & gunking about.
This trip was a total of 1,600+ miles on the pickup, averaging 16 mpg, $300 in gasoline, or maybe $500 if Macgregor had been in-tow ... Say hello to the new millenium ... Thanks again, for all the advice~!
