Owner of a 74 Hall GTC, and new member, here looking for other Hall owners to see about info sharing. This might be a long shot since there were only 200 of them made, but you never know.
For those not familiar with these rigs, they were produced by Hall Coach Company in So Cal in 1973 - early 74 - based on a Dodge B-300 extended wheelbase chassis and 360. These are one piece fiberglass units with a heavy outer shell, foam core with steel frame members and an innner glass liner. The Hall company was run by John Hall, the guy behind Revcon, and the son (or stepson) of the founder of Airstream - and the general design approach shows similarities with both makes. Aside from a couuple nice photos of one on Flikr, there is virtually no mention of them on the internet.