The further it goes, the stranger it gets :) ... Charging PUP battery from truck "house" battery is an option, but not a good one. Any charging-discharging results in 20-25% loss, i.e. to take 100 AH out you need to put 125 AH in. I would only consider this if no other options were available. And he would have to charge the truck house battery at home, rather than idling the truck for a couple of hours. Truck engine is a very inefficient charging machine, 2 hours of idling takes 2-3 gallons. And if generator is not allowed in the park, I pretty darn sure that either ranger or friendly neighbors will be quick to say something about that 4-wheel charger.
AGM batts... I have them because flooded don't work for me. But AGM are not all created equal.
Some - like Concorde - can take a very high charging current and it's recommended to charge them at high current to prolong their life (so high that solar can't provide that). Those you can charge fast, yes.
Some - like Deka or UPG - can't take more than 30 A initial current per 100 AH battery. Your truck might output 90 A, but you can only use 30A and only at the beginning charging stage, so there is no benefit there.
Some - like Optima bluetops - are not real deep-cycle but rather a hybrid of deep cycle and starter battery, there are pros and cons to this design.
