When you change from R-12 to R-134a (or any other type), you will
probably need to change the oil in the compressor to a different type
as well. Why not consider holding plates while you are changing things
around? I have two holding plate fridges and one low temp holding
plate freezer. They work well with about 3 hours of generator time per
day. A large inverter with suitable battery recharge capability could
replace the generator time, as could an engine driven compressor which
is even more efficient.