If you're at all handy they make a
bumper brace for RV's that should hold 4 bikes fine(I would relocate the spare tire). I removed my spare tire off of my tube bumper, bolted on a hitch receiver and have been hauling 2 bikes back there for the past 4 years with just the stock bumper. I regularly check the welds and it's held up fine.
I had a front hitch on my first tow vehicle and hated it, nothing worse than staring through bike handlebars on a trip.