I bought one for $70.00 and I plan to installation the tongue to keep the heat and exhaust away from the trailer.
The unit I have has a 3" vent and I assume could be installed inside if there were provisions for combustion air, but I think it is safer outside.
I plan to bend up a stainless box with a door to house it.
I have a technician that works for me who uses one for his barbecue (Professional) trailer and he tells me that pretty much you can set it and forget it.
He uses the hot water 100% (with no cold tempering) to get enough flow to turn on the flow switch an once set it stays pretty good.
I am still considering a way to mount it on the trailer, but so far I have not come up with what I am satisfied with.
I have tried this unit on the LP tank and garden hose and it works quickly and has controllable heat and I think that I will be as happy with it and any other and not worry about running out of hot water. When I have had a chance to mount it I will post some pictures here.
Personally I think a mounting box could be fabricated to mount it through the sidewall and vent out the top of the box with vents for combustion air and keep all of the business parts on the outside of the envelope. It is hard to find a good place for in on my trailer on the shell, thus the mounting on the tongue.
It could be mounted off the rear bumper, but with a long line from the LP tank and longer lines to the front bath and kitchen. I prefer to keep the lines as short as possible to get hot water fast and less loss.