Workhorse Trucks appear to have a promising solution. They build serial hybrid vans that have been purchased by UPS and FedEx. These are used for last mile delivery. They use a 60 KWH Panasonic battery and the 20 kw range extender from the i3. The range extender is too small for sustained driving at highway speeds but OK for a delivery truck. There's a news article out today that claims the smaller NGen van gets 40 MPGe in its 200 cubic foot configuration and 20 MPGe in its 450 cubic foot configuration.
I believe the vans share the same drive system as the Workhorse W-15 truck. If so, transferring some of the W-15 components into the van could make a very interesting base for a motorhome. The W-15 has dual 230 HP motors driving the front and rear axles. It is AWD with torque vectoring. The W-15 uses the larger BMW 1.5 liter turbo three cylinder range extender that will allow sustained operation at highway speeds at max load. The W-15 is able to offload up to 7.2 kws of power from its battery/range extender to 240 and 120 VAC outlets.
The Workhorse vans are currently being sold, the W-15 is due out to fleet users in 2018 and to private buyers in late 2019. Workhorse also plans heavier duty W-25 and W-35 pickups. Curiously, before being bought by AMP Workhorse used to build motorhomes, so it is possible there might be a Workhorse electric motor home in the future.
BTW, Workhorse is one of the five finalists for the USPS Next Generation Delivery Van project to replace their 180,000 mail deliver trucks which are falling apart and only average 9.6 mpg. The W-15 is claimed to be an outgrowth from this project. If they win, mailcarriers will have a truck that can go 0-60 in only five and half seconds and be equipped with air conditioning. I suspect the latter will prove to be more popular.