changing old 120v wiring in 54 year old trailer

To do it, your first choir is going to be removing all the siding, either inside or outside, your choice. 12GA romex should be sufficient for everything but maybe the roof AC. Remember not much inside that oldie will be 120VAC. If there are 120VAC fixtures, your best bet is to replace them all with 12VDC fixtures. Same for the gas lights, take them out.