Turning the truck around saves you wire for the length of the truck. Pretend that is 22 ft. So yes, just add 25ft of #6 wires (or whatever gauge) to be the extension cord, using only pin #4 and ground in a set of 7-pin connectors. Now all same except you don't have to turn the truck around.
The extra voltage drop will depend on the current drawn at the time for whatever you are doing. Use fatter wire for the 25 ft extension if it drops too far from what it is now (ASSuming that it is ok as is with truck turned)
The biggie is the heat mentioned. The furnace might be 7 amps right there increasing voltage drop from just lights. If it is just lights no problem, but what work is being done? Tools usually need 120v. If you have 120v then just run the converter to make the 12v.
The scenario is unclear. :)
BTW, I have run the truck for over an hour at idle with a 1000w inverter attached powering a 35 amp charger that needs 600w so I know that is an option. Long 120v extension cords are not nearly the problem that long 12v cords are for voltage drop.
