http://www.promasterforum.com/forum/showthread.php?t=36361&highlight=TopsiderAre the drain bolt and filter easily accessible ? Wondering why you wanted to go from the top?
On my Subaru wagon someone epoxied the drain bolt in, so I didn't want to mess with it. I bought a hand pump from west marine and that worked well to do a change from the top.
Not to mention the fact it's a good excuse to look things over. Checking for leaks, stray grease, torn boots, and loose components is a good way to avoid being stranded or gouged on the price of an emergency repair.I'm old and lazy but I have no problem reaching under the Promaster to remove the drain plug to insure all the old oil and crap gets drained out!