Sorry for delay is responding and many thanks for input from everybody.
I have used this set up before. I have been able to get 7 hours out of it in the past.
I am not using a humidifier. I think that I have probably run the battery down to a low state of charge in past camping usage as you describe. Last time I used it was 5 days in a row at a campground charging the battery up during the day. I lost ground each day as the battery came back with less and less charge at the end of the daytime charge cycle. I bet it gradually dug deeper and deeper into the battery charge level and maybe did permanent damage to the battery. The Booster Pac ES2500 has its own ac wall charger and circuitry that allows you to keep the battery charging 24/7. I think these engine starter batteries do not have the right design characteristics and are probably not optimized for a 2amp 8 hour draw.
Yes indeed current draw without humidifier is 1.4 Amps at 12v dc per SuperSleeper ResMed battery link. So it should be possible to get 10 hours out of it. But I think SuperSleeper is right I need a new bigger battery.
It is a MSW inverter but I am not running the humidifier.
So I will take your collective advice and get a ResMed DC converter and get a new battery.
So much for the Li-Ion 7.8 Ah camping battery packs I guess they will hardly run the machine for anything like long enough. These deep cycle abtteries are heavy so I am just going to have to lose weight and go out without the machine for a few nights.