Rv's and boats use Lithium Phosphate (Lifepo4) batteries which do not suffer thermal runaway. The less stable lithiums are of a different chemical makeup such as Lithium Cobalt. Although less stable they do hold more energy per pound/volume.
With some shopping and willingness on your end to learn and install I believe you could come close to buying a 400amp Lifepo4 pack and 400 watts of solar for what you would be paying for just the 400w solar option.
The suggestion of having them run wires ahead of time is spot on.