Yes, on everything suggested so far. I find that if the battery drains completely, the relay tends not to function. It needs some type of voltage source to the battery enough to engage the relay circuit. If the battery charges with the tow vehicle connected, then you shouldn't have a problem after, because the relay should get energized then.
Also, check the relay, there are fuses on each contact lead.
However, if your battery is not holding a charge that will be another issue.
1947 Curtis Wright
1958 Traveler 18'