Yes my gas millage on a long trip(Ontario to FL) was around 42MPG but when we tried the charge mode it went down to the low 30,s so it is not practical to ever use!
What we did do was find a old motel on way down that had doors to our room,on main floor, ran plug cord inside and charge battery over night.
It is expected that while in HV Charge mode that mpg will drop way down, but at the same time you are building EV range for later use, so it generally should work out to similar overall mpg, just slightly lower.
As an example, lets says you get 42 mpg and drive 42 miles in HV mode. You would use one gallon of gas to drive those 42 miles. Now lets say you do the same drive but this time in HV charge mode, and let's say you get 30 mpg while in in HV Charge mode. In that case you will use up one gallon of gas in only 30 miles. However during that time you will have been building EV range, let's say during those 30 miles running in HV Charge mode you build up 11 additional EV miles on top of whatever you had before. So at the 30 mile mark you shut off HV Charge and switch to EV and drive the next 11 miles in EV. So you have now gone 41 miles on one gallon of gas, or 41 mpg. Just slightly less efficient. This example is based on tests people have done on a repeated route like a daily commute, where they find that running in HV Charge is only slightly less efficient than running regular HV.
Okay so if it is slightly less efficient then why would you ever use it? You use it any time you want to have additional EV miles for whatever reason, generally related to improving the driving experience. It is most often used on trips, like when climbing grades when the gas engine can be pretty noisy in HV mode, especially with 0 EV range. So what some people do is if they know they don't have enough EV miles to make it up an upcoming hill climb in EV mode, then prior to getting to the hill they run in HV Charge to build up additional miles, so that when they climb the hill they can switch to EV and climb the hill quieter. Even if overall it is slightly less efficient it is worth it to them to reduce the noise and make the drive more enjoyable.
Another example would be if you were on a trip and stopped at a rest stop or for lunch or whatever, and when you left the rest stop you forgot to switch to HV mode, and by the time you realized this you used up all of your EV miles, which you had planned to use when you arrive at your destination city as you prefer to drive in EV mode around town. In this case you can use HV Charge to build up some EV miles to be used later. Again this is slightly less efficient, but then again so is driving at 70 mph vs driving at 65 mph, but at some point we make decisions that are based on comfort and convenience not solely on squeaking out maximum efficiency.