My setup.
I'm no expert but I'd say
don't even think about running a "T" into the van exhaust. I've only tore apart a Webasto but I bet the Espar is the same in that the internal van is plastic. Without the heater running the hot exhaust gases could backup into the unit and melt the fan.
Good news about the Espar muffler. With it installed you can't hear the "jet engine" noise the heater makes. It's very quiet.
Bad news. My Espar muffler leaks like a sieve.
In regards to Lens question,
I'm east of the Mississippi. Come on down to WV and I'll give you a hand. As an alternative, if you don't want to drive that far
, I'd suggest calling a local Freightliner (or other big rig) dealership and ask if they will do an install. These heaters are made for truck installs so if they don't do it I bet they could point you to someone who does.