OK...thanks for the additional info. My advice would be to replace everything with just a basic DC unit since your needs are modest and this is the cheapest way to go. I am a BIG fan of the Seafrost BD system
but it is not the cheapest one out there at about $1350. You can get a Waeco/Adler Barbour Cold Machine unit sufficient for your needs for less than $1000 with your choice of evaporator or cold plate.
No R12 new compressors may be sold today...so that is not an option. Your fridge guy MAY be able to purge your old lines and cold plate of R12 and install a new compressor with R134a refrigerant...but that is time consuming and expensive and if you are reasonably handy you do a whole new install for less. (It is not at all difficult ).
As to your batteries/alternator.
1. A new fridge unit will "cost" you about 60AH's a day which means that you need 120AH's of battery capacity JUST for the refrigeration and with a stock alternator as your only at anchor charge source..this means about 2 hours of running the engine daily for your fridge needs....+ whatever else you use.
2. It is unclear from your post what your total house bank AH capacity is...but I would suggest 300AH's or so should be sufficient for both refrigeration and general house needs for our casual usages. I do NOT suggest investing in a larger alternator/regulator as that is a big bill for a little use.
3. The Truecharge20 will take about 15 hours to 100% charge a 300 amp battery bank that has been 50% depleted. Suggest you leave it plugged in when not on the boat. otherwise it is likely you are fully charging your batteries only when you spend the weekend at the dock.
Hope this is helpful to you.