Yep, this will certainly work.
Flexible antennas on a portable radio are a weak solution at best. On an 800MHz or similar radio, they work pretty well because the ground plane presented by the radio is sufficient. As you drop down to UHF, it's still pretty good. Getting down to VHF, and the size of the radio makes it difficult to provide a proper ground plane under the antenna. Low band, forget about it. Low band HT's work, but not always well.
Getting a "real" antenna outside the vehicle will eliminate the weakness inherent in the flexible antenna, and it will also fix the big issue you are having. The truck cab is acting as a faraday cage, all the metal around you , metallic tinted windows, etc block the signal. A good external antenna will greatly improve your reception, and it will help your transmitted signal, even without an amp.
I've done this with VHF and UHF radios, and it works very well.
The big issue is the hassle of constantly connecting and disconnecting your radio, and potential damage to the connector. Most adapters that fit the top of the radios are intended for in shop testing, not long term every day use. You will risk flexing the antenna mount and causing issues with the connection between the mount and the circuit board. It's not uncommon for the solder joint to fail at this point on some radios. Also, RF connectors are usually rated for a certain number of mating cycles. After a while, they just wear out. Depending on the type and quality of the connector, this could range from a few hundred to a few thousand cycles before you'd likely see enough wear to cause problems.
Unfortunately Motorola doesn't make a vehicle adapter for the Waris (HT-1250 etc) line of radios that I'm aware of. They do for the Jedi and XTS line. They make switching back and forth as easy as sticking the radio in a docking station and locking the lever to hold the radio in. Nice solution, but not going to work with your current radio.
Likely it might be cheaper and easier to just get the proper mobile radio. A decent mobile VHF radio can be found used for anywhere from $100 on up. You can get new Icom VHF mobiles in the mid $200 range. I think if you are going to need a long term solution, getting a mobile radio is going to save you a lot of headache later.