Filling the GMC LPG tank with a 20 pounder. It works by having a pressure difference caused by the sun heating the small tank and the GMC tank being relatively cooler. Inverting the small tank causes it to transfer liquid making it much faster than transferring vapor. This takes 5 to 20 minutes to happen. I tell when it is done by feeling how the weight changes on the small tank and if it feels like there is liquid still inside. Not sure it would work with the new style bbq tanks with the float deal in them.
Filling a 20 pounder from my bulk tank. Once again it transfers because of a temperature/pressure difference. I do it in the morning as the smaller tanks drop temperature overnight more than the bulk tank. I've put small tanks in a tub of cold water to get it to transfer too. Really cheap to fill the coach and bbq tanks this way. Installed a valve in the bulk tank bottom to get liquid out.
Chuck K had a couple of rear Eldorado disk brake backing plates to modify to fit his GMC. He had a spare spindle I used to get a good fit.
Basically the plates needed to be opened up and a chamfer put on the spindle hole to match the larger GMC spindle.