Something you might want to try, if you want some really hot water.. A solar hot water bag. I bought 4. You can find them at wal-mart, in the camping section. Each bag holds 5 gallons of water. Let it set in the sun for about 3 hours, and voila! Then just dump the water into your bucket.
I'm a fan of solar shower bags myself; simple, nothing to wire or plumb. But on a cloudy, cold day you get a cold shower. They can be used alone and gravity fed or like mentioned above, you could transfer to your bucket.
If my water is cold I just boil a pot of water and dump it in. Warms up the whole thing.
I'm contemplating building a shower enclosure inside the van. The toilet would sit inside and act as a shower seat. The water would drain into a tank I'll install under the van. Grey water could be re-used for something...
Small heat exchanger, (buy or build one), plumbed inline with your heater hose. 12VDC on demand pump and some hose. You can have a hot shower until your truck runs out of gas! Built one for my Dodge. Stick one hose in the creek, and take a nice long shower. Need to install a valve to control flow through the exchanger. This will adjust the temperature.