They wont let you book room only without AP apart from DCR which anyone can book room only as the cost is for the cabin not per person.
Its definitely worth staying off site once to get AP - we have saved so much with it and definitely couldnt have done all the trips we have without it.
if you go at a quiet time you could book one night off site then hope you can get an onsite room once you have the AP. You are risking not being able to get anywhere though. I think some of the cheaper off site hotels have a cancellation policy where you dont pay until you leave and you can cancel up to 24hrs before. Im sure holiday Inn have diff rates and this is one option but you pay slightly more per night so you could book HI for few nights, arrive get AP, look into room only for later in stay, stay first night, at HI, then if youve managed to get room only on site cancel rest of nights and move hotel- check their website as it tells you different rates (you can even do it without breakfast which we found much better)