Hello Pat,
My plan is to pull a 1000 watt Honda generator behind me on a trailer. It takes 3 hours to charge my battery to 100% but like you I think charging it to 80% will be much quicker and more efficient. I'm no electrician but I would plug a power meter into an outlet then your charger to find the wattage of all the bikes. See if it is under the specs of your trucks outlet.
If I were to go on a trip with multiple riders I think I would bring my Honda 2000 generator along, it would likely handle 3 or 4 chargers at once.
Please let me know how things work out. Too bad your not closer to Colorado!
Take care,
Gerald