I have just used an extra car battery and diodes to drop volts on a few low volt runners. Ran all day on it on my VW van cargo floor and in a pickup bed.
Another year I had a big converter and the battery in the truck, but had to run the truck motor to last all day. I forget if I finally used the KW or 1033s.
A deep cycle battery helps too (I used one for the VW.. a 69 camper)
Note: trucks use 24v batteries, or two -12v in series.
I bet you have an old rheostat too