Thanks to all for your suggestions. I decided to proceed with my plan of using my 1.4KVA UPS in conjunction with 12VDC input from my truck's alternator as one of my backup power solutions. (the other is fixing my diesel generator).
Per advice in these newsgroups, I decided to modify my original plan to incorporate a battery isolator (none-1-ALL-2 switch rated for 200A continuous), and a trickle charger into my design. I will attach the isolator to the back of the UPS where I would have enough space to drill holes and neatly attach the switch and put the wires through the UPS cover.
The UPS and the wires leading to the pickup will be made of 3 gauge stranded wire. I will also use same wires to connect the UPS battery to a trickle charger by putting the isolator switch into the ALL position.
There is every reason to expect this to work well.
Total cost will be about $30 or so, spent on 40 ft of 3 ga wire and some cable lugs and such. The UPS (1400KVA Ferrups FE) I got for free.
i