The closest you can get to London without entering the LEZ is probably Sewardstone
http://www.leevalleypark.org.uk/en/cont ... stone.aspx I've never stayed there but have been into reception to enquire. I'm also not sure if they can accomodate RVs either (I'm still pulling a Tin Tent - it's a long story!) so please check. It also gets horrendously busy in the holidays and is seasonal.
Don't confuse this site with the similarly named Lee Valley Campsite at Pickets Lock. This one IS DEFINATELY inside the LEZ so be careful not to confuse as it could cost you £200 if you're an oil burner! They are both run by Lee Valley Council. The Pickets Lock site does have large 40ft RVs on it from time to time and they have one or two extra long hardstands too. Plenty of space for 30footers I'd say. It's a good site for London if you have wheels when you get there or there's a bus outside (might be seasonal though) but I wouldn't recommend walking from it as it involves a busy road, lots of industrial traffic and some not so desirable areas if you don't know the place (as with all city sites I'd say). The site is open all year except New Years Day (possibly Christmas Day).
Neither of these sites are picturesque but you wouldn't go to London for views of the hills anyway! But Pickets Lock is fine if you just need a base with decent facilities. Both are well maintained and staff friendly. There are some long-term big Hobby Vans and the like now and the warden said they'd relaxed the rules a little to accomadate contractors on the Olympics build but they seem to put them over the far side. Plus contractors are humans too and I enjoyed a beer and a shared TV outside vans during the World Cup so we're not talking undesireables here. In the nearfield bits there's a good mix of nationalities and rigs.
And for those of you worried about security, well it's a safe as anywhere really. No more no less. Barrier and locked night gate. I've been staying there at least once a month for a while and never had an incident except when a fox ate my rubbish which I'd stupidly left outside!
Finally (before you all nod off) I've never seen a 40footer leave this site and would imagine they have to let you out of the 'in' gate because the turn on the 'out' gate is tight. Other than that access is pretty easy even in large 40fters with a main road and about 1/2 mile from the North circular (the 3 lane section) and the bottom of the M11.