Verify fuel is being pumped by LP. Then check if fuel is coming out of IP: loosen a fuel line at injector and see if it weeps fuel after cranking it for a while. If no fuel feel for the fuel shut off solenoid operation.
Any codes? You said batteries are clean and connections look good make sure they are; side posts can be decieving especially passanger pos post take it apart and clean. Check the grounds and other end of battery cables. Also all fuses.
Other thing you could do is plug in block heater to make sure its warm enough to start too. Batteries should turn over pretty quick too. Recharge them if possible.
97 5spd K2500 Ext Cab short Bed ~160K miles.
TM, 3" downpipe & 4" exhaust, remote FSD, remote oilfilter, Gauges: EGT, Boost, Fuel Pressure, B&W Gooseneck Turnover ball, Prodigy Brake Controller. Hi-Temp Hydraulic Oil Cooler Lines.