That is a great price for a Trojan 150AH. I've seen them on some web sites selling for double that price.
Yes, having a set of back-up batteries is a good idea if the budget allows it. At the start of each work day, you will have a set of fully charged batteries.
I hear you on the $900 charge, that's a lot of coin every month.
A modified sine wave inverter will be about 20% less efficient as compared to a true sine wave inverter. Modified sine wave inverters may also generate a slight humming noise due to harmonics. Other than these 2 issues, the modified version will work OK on the fridge. Not the cleanest power, but will get the job done.
Yes, I will ask my neighbor tomorrow regarding his fridge/freezer T-Stat alarm. If the truck will have a data network interface device, there is a company that manufactures a Network Enabled Relay that can send out e-mails or text messages. Requires an RJ45 plug-in connection to a modem or a router. I believe it also works on iPhones. The relay contacts could be connected to an internal fridge T-Stat and provide you with alarm alerts based on the internal fridge temperature.
See link shown below for the Network Relay:
I designed and prototyped a low battery voltage monitor alarm circuit a few years ago. I was not able to locate a control panel that I could purchase for this application, so I built my own. The circuit is called a voltage comparator. Kinda of a pain to tweak the potentiometer (POT) and get the cutout voltage setting correct for the low voltage. I used my 12VDC power supply to tweak the POT and to adjust the cutout voltage. I set mine to alarm me at a 11.5 VDC cutout voltage. I built this comparator circuit panel for my solar panel application on my home where I use PV panels to provide emergency power for my 12 VDC battery back-up sump pump. I also use this control panel for emergency lighting in my home. When I loose commercial AC power due to a storm, I have selected ceiling fan lights in the home that turn ON within a few seconds on the loss of commercial power.
The cutout voltage on the VMA model could easily be wired to the above Network Relay to provide battery low voltage alarming status. The schematic shows a DC voltmeter. The picture does not have the voltmeter as it was my 1st prototype. Thus the only difference between the picture and the schematic.
Not sure where to obtain software for monitoring........ I'd have to research that.
See attached schematic, picture and parts list shown below: