I think even with restarting freezer twice a day in hot van, you are risking too high temperatures.
Get one of those thermometers with outside display to start with.
When I was restoring old Airstream trailer - the fridge was shot and replacement in direct size unavailable.
I put household, $120 fridge in it and wired to inverter.
The 36" tall fridge was taking very small current and I calculated that 100 amh battery would power it continuously for over 3hr.
Since I used the inverter on the road and never stop for longer than 3 hr, the fridge was working very well.
With today's technology you have lot of choices. You can add bigger battery bank and have freezer having power all the time, or have a timer that would run freezer 4-6 times a day to time it with generator run at certain time of a day.
The main idea is that generator recharge the batteries, what via inverter runs freezer 24hr a day.
Adding solar would be ice on the cake.
