The Fault is not in Our Stars but in Ourselves
The Original Poster (OP) said his vehicle's speed dropped to as low as 25 mph on the shallowest of grades. On an Interstate, with 70 mph speed limits being common, that's a 45 mph difference. Normal traffic will be going three times as fast as his 55 year old motor home is capable of on those grades. Bicycles and mopeds are banned on some Interstate Highways for precisely this reason. Then there's the fact that the OP stated that his vehicle started to get unstable above 50 mph or so. That in and of itself is VERY not good. If he can't control his vehicle at highway speeds, that vehicle has no place on the highway. The OP claimed it was a 'white knuckle ride' negotiating the flat terrain of LA, also not very good.
Finally, there is the fact that the OP apparently knows that all this is unsafe and asked for 'serious advice' and stated that he 'definitely needed trip advice.' I gave him my best advice. That advice was NOT based solely on his comfort or just for his benefit, but on the safety of the thousands and thousands of innocent people who would be forced to share the roads with no knowledge of how unsafe someone else's actions have made those roads. I stand by my comments that his vehicle has NO PLACE on the Interstate Highway System.
AZBuck
Band-Aids and Saying Good-Bye to an Old Friend
First, the people you are forcing to pass you on the right are not 'nut jobs'. They are simply trying to make the best of the no-win situation you have placed them in by insisting on traveling at well below highway speeds in a passing lane.
Second, your vehicle IS under-powered. The 1966 Ford F250 came with at least a 150 hp in-line 6 engine and could have as big as a 200 hp V8. Either one of those in good working order could push your motor home at well past 25 mph up the Grapevine, which at 5% grade is not as steep as is allowed on the Interstate System. That alone indicates that your engine is not in good working order.
Third, the 1965 and 1966 models introduced a new frame and far more complicated suspension system. While this new system allowed for a smoother ride, it did so at the expense of controllability if any component of that system degraded significantly. Your suspension has clearly degraded.
What I see in your posts is something that I experienced myself with my faithful old 1957 split-window SAAB 93. That is a vehicle that degraded so slowly over time that I never noticed how unsafe it had become little by little, year after year. Your motor home is now clearly (to an outside observer anyway) well past retirement age. It is probably not worth the cost of repairing/replacing the engine and suspension. I doubt that it is even worth the price of shipping it to Florida. I know it's hard, but I think it's time to say good-bye to this old friend and treat yourself to something newer and, more importantly, SAFER.
AZBuck