We use Webasto Air tops, Eberspacher is another.
They are excellent and work anywhere anytime, even while driving if you wish.
They might be expensive but compared to ALL your tools, hoses, chemicals and Truckmount, they protect an awful lot of your investment and if you sell the van in the future you can have it transferred for a couple of hundred £.
I have seen electric heaters fail and set fire in vans, don't do it, dont go the cheap route.
Like anything, you get what you pay for !.