I`m towing a 9200lb, 35 foot TT with an 02 V-10 Excursion with 4.30 gears. It did just fine in the mountains you are asking about. no problem holding 55-60 on the big grades! just let the motor rev, that`s where it makes it`s power. coming from a diesel the RPM`s may scare you, but the engine was designed to be spun up that high under load. I routinely wind mine up to 4k+ RPM on any hill that requires it. you will need 4.30 gears minimum for the weight you are asking about.
Good luck!
BTW, the spark plug issue was fixed in the early 02 model year. I have a late 02 and it has the PI heads (more threads per plug) the old heads only had 3 threads per plug.