Well, I would mount them inside the van somewhere, if for no other reason it will keep the batteries at a reasonable temperature. Outside they get too hit, or too cold. Inside is juuuuust right. hehe
Also, you want to mount the inverter as close to the batteries as possible, really close. You want the cable from the batteries to the inverter to be as short as possible, certainly no longer than 4 feet. The farther away you have it, the bigger the cables you will need to minimize voltage (and amps) drop. Basically, mount the inverter next to the batteries, or as close as practical, then run extension cords to the appliances.
In most cargo vans, especially the E350's, the aux batteries are simply mounted along the side wall somewhere out of the way. Most I've seen are mounted between the wheel well and the driver's seat, but anyplace they are out of the way would do fine. Again, though, because of having to run a cable from the isolator to the aux battery, I'd keep it near the floor instead of mounting it high. That way you can run the cable under the van and drill a hole in the floor (sealed well with silicone or something) for the cable, or run the cable through the firewall inside the van.
The way I did mine if the E350 was, I removed the passenger seat and built a box, a desk, really, that also served double duty as a battery box that covered the batteries up. I then mounted the inverter to the front of the box. You can see it here:
http://www.expeditersonline.com/dcforum/DCForumID16/1612.html#51
Doesn't show it very well, but should give you an idea. Not that I'm recommending that, unless you plan on making a box/desk. Along the side wall behind the seats is definitely the easiest mounting.
You'll want to secure the batteries so they don't move around or become a blunt projectile in an accident. Battery trays or some other fashioned bracket will do fine. I would recommend building a simple wooden box to cover them up, so as to eliminate risk of anything metal falling on them and shorting the batteries. The box can serve as a mount for the inverter, as well.
As for who can install it, most RV places could, and many of the better car audio installers can do that, as aux batteries are the life blood of today's new watt-sucking, mind-numbing, butt-thumping audio systems.
Ask around. If someone can't, or is too busy (as may very well be the case with an RV place), they will probably know someone who can do it for you. Ask an auto electric guy (alternator repair) if he can do it, or if he knows someone who can.