some bad news, you've been miss informed..
the th350C is not an overdrive transmission.. it is a th350 with a lock up converter ONLY...
you didn't say what your budget is.. so I'll go from cheapest to $$
first up is a 2004r this willbolt in place of a th350 and gives you overdrive, a stong rebuilt one is 900-1500.oo
next up is the 7004r these where and still areused in trucks the newer ones 4l60e are computer controlled, but the 700r4 are not..
this one gives you a 308 ish first gear.. and overdrive.. but the big gear splits, can cause issues when pulling a load, but with your 4:11 gears it be fine, this will require driveshaft mods, guess'n your motorhome has a 2 piece shaft, so this might just be a yoke change. the mount is farther back so you'll be moving the crossmember.. these are about the same cost to rebuild as the 200r4.. is a little stronger and better(for you) low first gear
next up would be the 4l60e but the controller is 400+ but is the same size as the 7004r(same tranny) and in newer trucks so finding a good used take out from a total shouldn't be hard (note after 99 they used the ls based engines)
next is the 4l80e think of this as a computer controlled overdrive th400..
same here used take outs (remember after 99 the ls engines are used different block bolt pattern)
lastly, the one thats on par in cost to most above after you add the mods and rebuild and new converter.. is the gearventers over/under drive
this bolts to your th350 and is an overdrive. but it allows you to split each gear, that in your case gives you 6 gears.. they are 2200-2400 but are basicly bulletproof,
hope this helps..