I live in an RV and was looking for the same thing for similar reasons, and the only thing that came close was the top of the line T Mobile plan. It's 95$ / month for the line, and allows for unlimited tethering / hotspot usage. They will throttle you only if a) you are over 50GB for the current period and b) if the tower you're on is currently in a state of high traffic. I have been over 50GB a couple times and didn't notice any slowdowns, so it appears the high traffic thing is for real. It's a lot for a phone line but it works well and if you're willing to spring for a cell booster / antenna it works even better. For me it's worth not having to worry about restrictions when I'm going to download something...
Be careful shopping around... lots of places offer "unlimited data", but they don't include hotspot usage (or tethering via USB). With those, you'll be required to install software on your phone to enable data usage, which will monitor the phone's connection to make sure you're not sharing it.
Good luck