If you plan to boon dock for extended periods, understand that your generator (assuming you have one) likely runs off propane which for people like me would be an issue. The generator itself is not a reliability issue. The matter at hand is that it's using up your propane. Once your propane is gone, everything inside the house shuts down, the fridge, the stove, the furnace, the hot water, and of coarse the generator, soon followed by your batteries. So the best you can hope for then is charging your batteries while bee-lining it to a propane filling station.
With the Ford chassis, you'll have a 55 gallon drum worth of gasoline which the gasoline generator will have it at it's disposal for all but the last 1/4 tank so theoretically you have as much as 41 gallons of gas to support the generator.
We tow a vehicle so we gas up (and top off 6 gallons more) our Ford gasoline powered motor home and drive to a primitive campground where it could sit put for a week, so 41 gallons of gas is good to have around to support the generator which we'll use to charge the batteries and generate hot water via electricity at the same time. With charged batteries and adequate propane, we can run the furnace at night without concerns over battery and/or propane depletion. Only once did we run our gas tank so low that the generator stopped working. But it was time to move on anyway, yet we still had propane for much of the basics.
On another matter over the 2007 L5 Ram/Freightliner diesel chassis, I have read it's load is rated less than current day. As a result, it has handling issues. If the previous owner did not make improvements to the suspension, you might find yourself investing further to help the rig handle better. Mind you the Ford of the same era isn't any different. I had to invest too in our 2007 E350 chassis.
With everything mentioned, I did want to say that if that 70K View for $40,000 is in decent shape, it seems a fair deal for both parties.