You can convert a TH350C w/Lockup TC into a TH350 Non-Lockup TC Trans but, not vice versa.
The TH350C's Lockup TC would only drop your Engine's cruising speed about 150-200rpm.
If your Motor Home has a HD 350 Trans it's most likely a TH375B. I'm surprised GM didn't use a TH400 Trans in a C30 MH.
A better option would be a 4L80E Trans or GV/Gear Vendor OD/OverDrive unit. These would give you about 25% rpm drop in OD which would be Ok on flat/level road.
A vehicle like that should have come with a SBC400 or BBC454 Engine & TH400 Trans.