How are you connecting the truck electrical system to the RV battery? If it's through the 7-pin trailer connector, it's not in the least bit surprising you're seeing very little charge current as there is sufficient voltage drop between the two to quite effectively limit the charge current.
If you connect the two as directly as possible with as heavy wire as practical, you should see somewhat better performance. The voltage regulator system in the truck may need to see the "low" RV battery (i.e. have the system voltage pulled down some) to kick up to a higher charging voltage; modern regulators are not the simple-minded fixed voltage output systems of earlier times.
Good, heavy-duty jumper cables might work reasonably well. Many jumper cable sets sold today are not good, heavy-duty ones, with smallish aluminum ("CCA," copper clad aluminum) wires in unnecessarily thick rubber insulation so they look more impressive at first glance.
