I don't know how many people have this issue, but our 38J was designed for the "short queen" mattress. We had the dealer upgrade to the sleep number bed and they installed the full queen mattress. We initially thought this was great (no feet hanging off the end of the bed), but had some problems when retracting the bed slide. My solution was to make a sliding support for the end of the air mattress. This keeps the mattress from sagging and getting stuck between the bed and the drawers.
I used 1/4" lauan plywood. (I thought about using heavy sheet metal, but didn't have a good way of making a good right angle bend that wide, and was worried about sharp edges) I first cut a piece as wide as the mattress overhang and slightly shorter than the bed width. Then a second piece was cut the same length, but about 6" high for the end. These two were connected together using angle brackets. Use something like tee nuts on the back side of the lauan, otherwise screws will protrude through (plus 1/4" lauan isn't strong enough to hold wood screws). To mount the slide support to the bed, I used 4 1/4" carriage bolts (I had to cut to the the correct length so I wouldn't have too much poking through). I cut slots in the slide support the width of the the carriage bolts so the bolts could freely slide in the slots. Then I mounted the lauan support using the carriage bolts with large fender washers on both sides of the lauan. To cover this ugly mess, I bought the cheapest mattress cover from WalMart and attached it to the lauan using 1/4" staples.
We've been using this for over 4 years so far without a mishap. We are careful not to sit on the end of the bed since I don't trust 1/4" lauan for too much support. When retracting the slide, it slides in effortlessly without assistance, and usually the mattress will push it back out when extending the slide. After long periods of storage, or when the mattress is completely deflated (like when crossing high mountains), we will have to manually pull it out, or wait until we pump the mattress back up.
Side view - support slid into travel position
Top of support
Support Extended
Support in travel position
Bed with slide extended
Bed with slide retracted. Notice slight compressing of air mattress.