Those "jump packs" are two sealed 12AH batteries (at most) and the assorted charger and cables. They will only last a short while and will then not do what you want.
I'd suggest you either visit a junkyard or pony up the $80 for a new auto battery. When you get some chips, head over to Costco or Sams and get two 6V's. The golf cart batteries are durable and will allow for some abuse. The jump pack or auto battery won't withstand but a few beatings.
FWIW, I have many chargers and solar on the RV. For auto and tractor batteries I over-Winter, they are charged with a plug-in wall transformer a few times over the Winter. That keeps them desulfated and adds to the longevity. If you have a battery on the tongue in a cold climate and access to shore power, you can run one of these small transformers as a 24/7 float charger. Observe voltages.
