Everything I read says to advance your timing at high altitude. Here is one article.
http://rmrclc.com/WaltsWorkBench/10-2000 Attitudes about altitude.html#:~:text=Your ignition timing is probably,about 8 to 10 degrees.
Your post got me looking for a front window covering and I saw this..... would look good I think.
https://www.ebay.com/itm/Mountain-Scene-4-River-and-Trees-Truck-Tailgate-Vinyl-Graphic-Decal-Wrap/332334509763?hash=item4d60ae2ec3:m:mRg0QKNQc-dfNlnHZ7SWZVg
Try this....remove the two hoses going to the heater and join them together.....then pressurize the cooling system and see if it holds pressure.If pressure holds its a good chance it IS just a leaking heater core. I'm thinking the radiator cap should have opened and blown off steam before the heater core gave if the core was still in good shape.
Go to and join this site...https://www.yotatech.com/forums/f116/ ...lots of gearheads there...the same motors that our RVs have...join and type in what you posted here.