Why would you go all the way North to 88 or 50? 120 Through Yosemite is MUCH more direct, going up to 88 is 90 miles further!!. The only place you have to watch is just past Moccasin stay on the highway taking New Priest Grade, designed for busses and RVs. DO NOT take Old Priest Grade. This will be the worst of it and the 351 will do just fine.
Or if you don't want to do this section, drop to 140 and take 140 in to Yosemite and 120 out, only 30 miles longer than take 120 all the way, and still 60 miles shorter than 88. 140 never gets above 4000 ft. and is pretty uneventful. There is a one lane section around a rock slide, but you are short enough. The rest of 120 is not a big deal. Very picturesque, especially the Yosemite high country, stop and take pictures!. While Tioga pass is high in elevation it is not overly steep. Some 8% going down to 395, just gear down and enjoy the sights. I have done it in a truck camper and towing a 5200lb trailer with a 4.6 Explorer Once at 395, Mammoth Lakes is a short drive South
Like skip said, your biggest issue is going to be the pin weight which is typically 20% vs a pull behind usually at 13%. Good thing is a 5th wheel will not cause the same sway problems a pull behind can. From what I could find, the 93 XL and XLT had max payloads around 1400lbs. Options, passengers and stuff eat into that. Just keep your packing on the light side, drop the trailer on and see how it looks. If the springs are bottomed out, don't do it.
Enjoy! I wish I was going to the Eastern Sierras :(
