13'6 is legal height max. Over that and overpasses become an issue. Nearby on interstate 40. Many and i mean many are posted 13'6. you get to 14' and your air conditioners will meet asphalt and natural skylights you will have.
i guess you could get off every exit and get back on to avoid the lower overpasses
With a car, you should be able to travel on CA1 all of the way. I particularly liked the route from San Simion up to Monterey, and from Eureka, CA to Astoria, WA. Really, the only parts of that trip I would avoid with the type of drive you are thinking of would be the parts through the cities like LA and SF.
That should be the primary, if not the only reason for any type of fire extinguisher in most of our RVs. Years ago I was a volunteer firefighter and one thing that it taught me was getting everyone safe is the only important thing with most fires.
You could be closer by just weighing at the nearest truck scale like most larger truck stops have and then use the axle weight ratings to base you inflation on. If you do that I'd still run inflation pressures to the high side, but you can divide axle weights by half, keeping in mind that your weights probably aren't exactly half on each side. Remember that even when you get weights for individual wheels you still need to run the inflation at the same pressure for all tires on the same axle, basing that on the heaviest wheel.
Running at max inflation probably isn't unsafe but it may cause a stiffer ride and possibly uneven tire wear.